When running testcases libgomp.fortran/examples-4/declare_target-{1,2}.f90 on
an RTX A2000 (sm_86) with driver 510.60.02 and with GOMP_NVPTX_JIT=-O0 I run
into:
...
FAIL: libgomp.fortran/examples-4/declare_target-1.f90 -O0 \
-DGOMP_NVPTX_JIT=-O0 execution test
FAIL: libgomp.fortran/examples-4/declare_target-2.f90 -O0 \
-DGOMP_NVPTX_JIT=-O0 execution test
...
Fix this by further limiting recursion depth in the test-cases for nvptx.
Furthermore, make the recursion depth limiting nvptx-specific.
Tested on x86_64 with nvptx accelerator.
libgomp/ChangeLog:
2022-04-01 Tom de Vries <tdevries@suse.de>
* testsuite/libgomp.fortran/examples-4/declare_target-1.f90: Define
and use REC_DEPTH.
* testsuite/libgomp.fortran/examples-4/declare_target-2.f90: Same.
When running test-case libgomp.oacc-c-c++-common/vector-length-128-7.c on an
RTX A2000 (sm_86) with driver 510.60.02 I run into:
...
FAIL: libgomp.oacc-c/../libgomp.oacc-c-c++-common/vector-length-128-7.c \
-DACC_DEVICE_TYPE_nvidia=1 -DACC_MEM_SHARED=0 -foffload=nvptx-none -O0 \
output pattern test
...
The failing check verifies the launch dimensions:
...
/* { dg-output "nvptx_exec: kernel main\\\$_omp_fn\\\$0: \
launch gangs=1, workers=8, vectors=128" } */
...
which fails because (as we can see with GOMP_DEBUG=1) the actual num_workers
is 6:
...
nvptx_exec: kernel main$_omp_fn$0: launch gangs=1, workers=6, vectors=128
...
This is due to the result of cuOccupancyMaxPotentialBlockSize (which suggests
'a launch configuration with reasonable occupancy') printed just before:
...
cuOccupancyMaxPotentialBlockSize: grid = 52, block = 768
...
[ Note: 6 * 128 == 768. ]
Fix this by updating the check to allow num_workers in the range 1 to 8.
Tested on x86_64 with nvptx accelerator.
libgomp/ChangeLog:
2022-04-01 Tom de Vries <tdevries@suse.de>
* testsuite/libgomp.oacc-c-c++-common/vector-length-128-7.c: Fix
num_workers check.
When building an nvptx offloading configuration on openSUSE Leap 15.3, the
site script /usr/share/site/x86_64-unknown-linux-gnu is activated, setting
libexecdir to ${exec_prefix}/lib rather than ${exec_prefix}/libexec:
...
| # If user did not specify libexecdir, set the correct target:
| # Nor FHS nor openSUSE allow prefix/libexec. Let's default to prefix/lib.
|
| if test "$libexecdir" = '${exec_prefix}/libexec' ; then
| libexecdir='${exec_prefix}/lib'
| fi
...
However, in libgomp libgomp/plugin/configfrag.ac we hardcode libexec:
...
# Configure additional search paths.
if test x"$tgt_dir" != x; then
offload_additional_options="$offload_additional_options \
-B$tgt_dir/libexec/gcc/\$(target_alias)/\$(gcc_version) \
-B$tgt_dir/bin"
...
Fix this by using /$(libexecdir:\$(exec_prefix)/%=%)/ instead of /libexec/.
Tested on x86_64-linux with nvptx accelerator.
libgomp/ChangeLog:
2022-03-28 Tom de Vries <tdevries@suse.de>
* plugin/configfrag.ac: Use /$(libexecdir:\$(exec_prefix)/%=%)/
instead of /libexec/.
* configure: Regenerate.
When a display manager is running on an nvidia card, all CUDA kernel launches
get a 5 seconds watchdog timer.
Consequently, when running the libgomp testsuite with nvptx accelerator and
GOMP_NVPTX_JIT=-O0 we run into a few FAILs like this:
...
libgomp: cuStreamSynchronize error: the launch timed out and was terminated
FAIL: libgomp.oacc-c/../libgomp.oacc-c-c++-common/parallel-dims.c \
-DACC_DEVICE_TYPE_nvidia=1 -DACC_MEM_SHARED=0 -foffload=nvptx-none -O0 \
execution test
...
Fix this by scaling down the failing test-cases by default, and reverting to
the original behaviour for GCC_TEST_RUN_EXPENSIVE=1.
Tested on x86_64-linux with nvptx accelerator.
libgomp/ChangeLog:
2022-03-25 Tom de Vries <tdevries@suse.de>
PR libgomp/105042
* testsuite/libgomp.oacc-c-c++-common/parallel-dims.c: Reduce
execution time.
* testsuite/libgomp.oacc-c-c++-common/vred2d-128.c: Same.
* testsuite/libgomp.oacc-fortran/parallel-dims.f90: Same.
gcc/lto/ChangeLog:
PR middle-end/104285
* lto-partition.cc (maybe_rewrite_identifier): Use get_identifier
for the returned string to be usable as hash key.
(validize_symbol_for_target): Hence, use return value directly.
(privatize_symbol_name_1): Track maybe_rewrite_identifier renames.
* lto.cc (offload_handle_link_vars): Move function up before ...
(do_whole_program_analysis): Call it after static renamings.
(lto_main): Move call after static renamings.
libgomp/ChangeLog:
PR middle-end/104285
* testsuite/libgomp.c++/target-same-name-2-a.C: New test.
* testsuite/libgomp.c++/target-same-name-2-b.C: New test.
* testsuite/libgomp.c++/target-same-name-2.C: New test.
* testsuite/libgomp.c-c++-common/target-same-name-1-a.c: New test.
* testsuite/libgomp.c-c++-common/target-same-name-1-b.c: New test.
* testsuite/libgomp.c-c++-common/target-same-name-1.c: New test.
Consider this code (with N defined to 1024):
...
float v = 0.0;
#pragma omp target map(tofrom: v)
#pragma omp parallel for simd
for (int i = 0 ; i < N; i++)
{
#pragma omp atomic update
v = v + 1.0;
}
...
It hangs when executing on target board unix/-foffload=-misa=sm_75, using
drivers 470.103.01 and 510.54 on a T400 board (sm_75).
I'm tentatively identifying the problem as a bug in -muniform-simt for
architectures that support Independent Thread Scheduling (sm_70 and later).
The problem -muniform-simt is trying to address is to make sure that a
register produced outside an openmp simd region is available when used in any
lane inside an simd region.
The solution is to, outside an simd region, execute in all warp lanes, thus
producing consistent values in result registers in each warp thread.
This approach doesn't work when executing in all warp lanes multiplies the
side effects from 1 to 32 separate side effects, which is the case for atomic
insns. So atomic insns are rewritten to execute only in lane 0, and if
there are any results, those are propagated to the other threads in the warp.
[ And likewise for system calls malloc, free, vprintf. ]
Now, consider a non-atomic update: ld, add, store. The store has side
effects, are those multiplied or not?
Pre-sm_70 we can assume that at the end of an SIMT region, any divergent
control flow has reconverged, and we have a uniform warp, executing in lock
step. So:
- the load will load the same value into the result register across the warp,
- the add will write the same value into the result register across the warp,
- the store will write the same value to the same memory location, 32 times,
at once, having the result of a single store.
So, no side-effect multiplication (well, at least that's the observation).
Starting sm_70, the threads in a warp are no longer guaranteed to reconverge
after divergence. There's a "Convergence Optimizer" that can can identify
that it is safe for a warp to reconverge, but that works only as long as the
code does not contain "synchronizing operations".
Consequently, the ld, add, store sequence can be executed by a non-uniform
warp, which means the side effects can have multiplied, and the registers are
no longer guarantueed to be in sync.
The atomic update in the example above is translated using an atom.cas loop,
which means that we have divergence (because only one thread is allowed to
succeed at a time) and the "Convergence Optimizer" doesn't reconverge probably
because the atom.cas counts as a "synchronizing operation". So, it seems
plausible that the root cause for the mentioned hang is the problem described
above.
Fix this by adding an explicit warp sync at simt exit.
Note that we're assuming here that the warp will stay uniform until the next
SIMT region entry.
Tested on x86_64 with nvptx accelerator.
gcc/ChangeLog:
2022-03-09 Tom de Vries <tdevries@suse.de>
PR target/104916
PR target/104783
* config/nvptx/nvptx.md (define_expand "omp_simt_exit"): Emit warp
sync (or uniform warp check for mptx < 6.0).
libgomp/ChangeLog:
2022-03-15 Tom de Vries <tdevries@suse.de>
PR target/104916
PR target/104783
* testsuite/libgomp.c/pr104783-2.c: New test.
gfc_omp_predetermined_sharing cases the associate-name pointer variable
to be OMP_CLAUSE_DEFAULT_FIRSTPRIVATE, which is fine. However, the associated
selector is shared. Thus, the target of associate-name pointer should not get
copied. (It was before but because of gfc_omp_privatize_by_reference returning
false, the selector was not only wrongly copied but this was also not done
properly.)
gcc/fortran/ChangeLog:
PR fortran/103039
* trans-openmp.cc (gfc_omp_clause_copy_ctor, gfc_omp_clause_dtor):
Only privatize pointer for associate names.
libgomp/ChangeLog:
PR fortran/103039
* testsuite/libgomp.fortran/associate4.f90: New test.
Consider test-case pr104952-1.c, included in this commit, containing:
...
#pragma omp target map(tofrom:result) map(to:arr)
#pragma omp simd reduction(||: result)
...
When run on x86_64 with nvptx accelerator, the test-case either aborts or
hangs.
The reduction clause is translated by the SIMT code (active for nvptx) as a
butterfly reduction loop with this butterfly shuffle / update pair:
...
D.2163 = D.2163 || .GOMP_SIMT_XCHG_BFLY (D.2163, D.2164)
...
in the loop body.
The problem is that the butterfly shuffle is possibly not executed, while it
needs to be executed unconditionally.
Fix this by translating instead as:
...
D.tmp_bfly = .GOMP_SIMT_XCHG_BFLY (D.2163, D.2164)
D.2163 = D.2163 || D.tmp_bfly
...
Tested on x86_64-linux with nvptx accelerator.
gcc/ChangeLog:
2022-03-17 Tom de Vries <tdevries@suse.de>
PR target/104952
* omp-low.cc (lower_rec_input_clauses): Make sure GOMP_SIMT_XCHG_BFLY
is executed unconditionally.
libgomp/ChangeLog:
2022-03-17 Tom de Vries <tdevries@suse.de>
PR target/104952
* testsuite/libgomp.c/pr104952-1.c: New test.
* testsuite/libgomp.c/pr104952-2.c: New test.
On Thu, Nov 11, 2021 at 02:14:05PM +0100, Thomas Schwinge wrote:
> There appears to be yet another issue: there still are quite a number of
> 'FAIL: libgomp.c/places-10.c execution test' reports on
> <gcc-testresults@gcc.gnu.org>. Also in my testing testing, on a system
> where '/sys/devices/system/node/online' contains '0-1', I get a FAIL:
>
> [...]
> OPENMP DISPLAY ENVIRONMENT BEGIN
> _OPENMP = '201511'
> OMP_DYNAMIC = 'FALSE'
> OMP_NESTED = 'FALSE'
> OMP_NUM_THREADS = '8'
> OMP_SCHEDULE = 'DYNAMIC'
> OMP_PROC_BIND = 'TRUE'
> OMP_PLACES = '{0,2,4,6,8,10,12,14,16,18,20,22,24,26,28,30},{FAIL: libgomp.c/places-10.c execution test
I've finally managed to debug this (by dumping used /sys/ files from
an affected system in Fedora build system, replacing /sys/ with /tmp/
in gcc sources and populating there those files), I think following patch
ought to fix it.
2022-03-18 Jakub Jelinek <jakub@redhat.com>
* config/linux/affinity.c (gomp_affinity_init_numa_domains): Move seen
variable next to pl variable.
This patch fixes a small bug in the omp_set_num_teams implementation.
libgomp/ChangeLog:
* fortran.c (omp_set_num_teams_8_): Call omp_set_num_teams instead of
omp_set_max_active_levels.
* testsuite/libgomp.fortran/icv-8.f90: New test.
It's an orthogonal concern why these diagnostics do appear at all for
non-offloaded OpenACC constructs (where they're not relevant at all); PR90115.
Depending on how 'assert' is implemented, it may cause temporaries to be
created, and/or may lower into 'COND_EXPR's, and
'gcc/gimplify.cc:gimplify_cond_expr' uses 'create_tmp_var (type, "iftmp")'.
Fix-up for commit 11b8286a83
"[OpenACC privatization] Largely extend diagnostics and
corresponding testsuite coverage [PR90115]".
PR testsuite/102841
libgomp/
* testsuite/libgomp.oacc-c-c++-common/host_data-7.c: Adjust.
Currently in OpenACC 'kernels' decomposition, there is special handling of
'GOMP_MAP_FORCE_TOFROM', documented to be done to avoid "internal compiler
errors in later passes". For performance reasons, the current repetitive
to/from device copying for every region is not ideal, compared to using
'present' clauses, as done for almost all other 'GOMP_MAP_*'. Also, the
current special handling (incomplete, evidently) is the reason for the PR104892
misbehavior. For PR100280 etc. we've resolved all such known ICEs -- removing
the special handling for 'GOMP_MAP_FORCE_TOFROM' now resolves PR104892.
PR middle-end/100280
PR middle-end/104892
gcc/
* omp-oacc-kernels-decompose.cc (omp_oacc_kernels_decompose_1):
Remove special handling of 'GOMP_MAP_FORCE_TOFROM'.
gcc/testsuite/
* c-c++-common/goacc/kernels-decompose-2.c: Adjust.
* c-c++-common/goacc/kernels-decompose-pr100400-1-1.c: Likewise.
* c-c++-common/goacc/kernels-decompose-pr100400-1-2.c: Likewise.
* c-c++-common/goacc/kernels-decompose-pr100400-1-3.c: Likewise.
* c-c++-common/goacc/kernels-decompose-pr100400-1-4.c: Likewise.
* c-c++-common/goacc/kernels-decompose-pr104061-1-1.c: Likewise.
* c-c++-common/goacc/kernels-decompose-pr104061-1-2.c: Likewise.
* c-c++-common/goacc/kernels-decompose-pr104061-1-3.c: Likewise.
* c-c++-common/goacc/kernels-decompose-pr104061-1-4.c: Likewise.
* c-c++-common/goacc/kernels-decompose-pr104132-1.c: Likewise.
* c-c++-common/goacc/kernels-decompose-pr104133-1.c: Likewise.
* c-c++-common/goacc/kernels-decompose-pr104774-1.c: Likewise.
* gfortran.dg/goacc/classify-kernels.f95: Likewise.
* gfortran.dg/goacc/kernels-decompose-2.f95: Likewise.
libgomp/
* testsuite/libgomp.oacc-c-c++-common/declare-vla.c: Adjust.
* testsuite/libgomp.oacc-c-c++-common/default-1.c: Likewise.
* testsuite/libgomp.oacc-c-c++-common/kernels-decompose-1.c:
Likewise.
* testsuite/libgomp.oacc-c-c++-common/kernels-reduction-1.c:
Likewise.
* testsuite/libgomp.oacc-c-c++-common/parallel-dims.c: Likewise.
* testsuite/libgomp.oacc-fortran/asyncwait-1.f90: Likewise.
* testsuite/libgomp.oacc-fortran/kernels-reduction-1.f90:
Likewise.
Document a few examples of the status quo.
PR middle-end/104892
libgomp/
* testsuite/libgomp.oacc-c-c++-common/kernels-decompose-1.c: Point
to PR104892.
* testsuite/libgomp.oacc-c-c++-common/default-1.c: Likewise,
enable '--param=openacc-kernels=decompose' and adjust.
* testsuite/libgomp.oacc-c-c++-common/kernels-reduction-1.c:
Likewise.
* testsuite/libgomp.oacc-c-c++-common/parallel-dims.c: Likewise.
* testsuite/libgomp.oacc-fortran/kernels-reduction-1.f90:
Likewise.
... like in recent commit 9b32c1669a
"OpenACC 'kernels' decomposition: Mark variables used in synthesized
data clauses as addressable [PR100280]". Otherwise, we may run into
'gcc/omp-low.cc:lower_omp_target':
13125 else if (is_gimple_reg (var))
13126 {
13127 gcc_assert (offloaded);
PR middle-end/100280
PR middle-end/104086
gcc/
* omp-oacc-kernels-decompose.cc (omp_oacc_kernels_decompose_1):
Mark variables used in 'present' clauses as addressable.
* omp-low.cc (scan_sharing_clauses) <OMP_CLAUSE_MAP>: Gracefully
handle duplicate 'OMP_CLAUSE_MAP_DECL_MAKE_ADDRESSABLE'.
gcc/testsuite/
* c-c++-common/goacc/kernels-decompose-pr104086-1.c: Adjust,
extend.
libgomp/
* testsuite/libgomp.oacc-c-c++-common/declare-vla-kernels-decompose-ice-1.c:
Merge this...
* testsuite/libgomp.oacc-c-c++-common/declare-vla-kernels-decompose.c:
..., and this...
* testsuite/libgomp.oacc-c-c++-common/declare-vla.c: ... into
this, and adjust.
* testsuite/libgomp.oacc-c-c++-common/kernels-decompose-1.c:
Extend.
1. Thomas reported in
https://gcc.gnu.org/pipermail/gcc-patches/2022-January/589039.html
that this testcase is randomly failing. The problem was fixed pool
size which was exhausted when there were a lot of threads. Fixed it
by removing pool_size trait which causes default pool size to be used
which should be big enough.
2. Array indices have been changed to check the last element in the
array.
3. Remove a redundant assignment and move some code to better match
C testcase.
libgomp/ChangeLog:
* testsuite/libgomp.fortran/allocate-1.f90: Remove pool_size
trait. Test last index in w and v array. Remove redundant
assignment to V(1). Move alignment checks at the end of
parallel region.
I ran into a hang for this code:
...
#pragma omp target map(tofrom: counter_N0)
#pragma omp simd
for (int i = 0 ; i < 1 ; i++ )
{
#pragma omp atomic update
counter_N0 = counter_N0 + 1 ;
}
...
This has to do with the nature of -muniform-simt. It has two modes of
operation: inside and outside an SIMT region.
Outside an SIMT region, a warp pretends to execute a single thread, but
actually executes in all threads, to keep the local registers in all threads
consistent. This approach works unless the insn that is executed is a syscall
or an atomic insn. In that case, the insn is predicated, such that it
executes in only one thread. If the predicated insn writes a result to a
register, then that register is propagated to the other threads, after which
the local registers in all threads are consistent again.
Inside an SIMT region, a warp executes in all threads. However, the
predication and propagation for syscalls and atomic insns is also present
here, because nvptx_reorg_uniform_simt works on all code. Care has been taken
though to ensure that the predication and propagation is a nop. That is,
inside an SIMT region:
- the predicate evalutes to true for each thread, and
- the propagation insn copies a register from each thread to the same thread.
That works fine, until we use -mptx=6.0, and instead of using the deprecated
warp propagation insn shfl, we start using shfl.sync:
...
@%r33 atom.add.u32 _, [%r29], 1;
shfl.sync.idx.b32 %r30, %r30, %r32, 31, 0xffffffff;
...
The shfl.sync specifies a member mask indicating all threads, but given that
the loop only has a single iteration, only thread 0 will execute the insn,
where it will hang waiting for the other threads.
Fix this by predicating the shfl.sync (and likewise, bar.warp.sync and the
uniform warp check) such that it only executes outside the SIMT region.
Tested on x86_64 with nvptx accelerator.
gcc/ChangeLog:
2022-03-08 Tom de Vries <tdevries@suse.de>
PR target/104783
* config/nvptx/nvptx.cc (nvptx_init_unisimt_predicate)
(nvptx_output_unisimt_switch): Handle unisimt_outside_simt_predicate.
(nvptx_get_unisimt_outside_simt_predicate): New function.
(predicate_insn): New function, factored out of ...
(nvptx_reorg_uniform_simt): ... here. Predicate all emitted insns.
* config/nvptx/nvptx.h (struct machine_function): Add
unisimt_outside_simt_predicate field.
* config/nvptx/nvptx.md (define_insn "nvptx_warpsync")
(define_insn "nvptx_uniform_warp_check"): Make predicable.
libgomp/ChangeLog:
2022-03-10 Tom de Vries <tdevries@suse.de>
* testsuite/libgomp.c/pr104783.c: New test.
... by generalizing the existing 'gcc/omp-low.cc:task_shared_vars'.
Fix-up for commit 9b32c1669a
"OpenACC 'kernels' decomposition: Mark variables used in
synthesized data clauses as addressable [PR100280]".
PR middle-end/100280
PR middle-end/104132
PR middle-end/104133
gcc/
* omp-low.cc (task_shared_vars): Rename to
'make_addressable_vars'. Adjust all users.
(scan_sharing_clauses) <OMP_CLAUSE_MAP> Use it for
'OMP_CLAUSE_MAP_DECL_MAKE_ADDRESSABLE' DECLs, too.
gcc/testsuite/
* c-c++-common/goacc/kernels-decompose-pr104061-1-3.c: Adjust.
* c-c++-common/goacc/kernels-decompose-pr104061-1-4.c: Likewise.
* c-c++-common/goacc/kernels-decompose-pr104132-1.c: Likewise.
* c-c++-common/goacc/kernels-decompose-pr104133-1.c: Likewise.
libgomp/
* testsuite/libgomp.oacc-c-c++-common/kernels-decompose-1.c:
Extend.
When running with target board unix/-foffload=-mptx=3.1, we run into:
...
lto1: error: PTX version (-mptx) needs to be at least 4.2 to support \
selected -misa (sm_53)^M
mkoffload: fatal error: x86_64-pc-linux-gnu-accel-nvptx-none-gcc returned \
1 exit status^M
compilation terminated.^M
...
FAIL: libgomp.c/declare-variant-3-sm53.c (test for excess errors)
...
Fix this by adding -foffload=-mptx=_ in the libgomp.c/declare-variant-3-sm*.c
test-cases.
Tested on x86_64 with nvptx accelerator.
libgomp/ChangeLog:
2022-02-28 Tom de Vries <tdevries@suse.de>
* testsuite/libgomp.c/declare-variant-3-sm30.c: Add -foffload=-mptx=_.
* testsuite/libgomp.c/declare-variant-3-sm35.c: Same.
* testsuite/libgomp.c/declare-variant-3-sm53.c: Same.
* testsuite/libgomp.c/declare-variant-3-sm70.c: Same.
* testsuite/libgomp.c/declare-variant-3-sm75.c: Same.
* testsuite/libgomp.c/declare-variant-3-sm80.c: Same.
Add openmp test-cases that test the omp declare variant construct:
...
#pragma omp declare variant (f30) match (device={isa("sm_30")})
...
using the available nvptx isas.
Only the one for sm_30 is a dg-do run test-case, the other ones are dg-do
link.
Tested on x86_64 with nvptx accelerator.
libgomp/ChangeLog:
2022-02-24 Tom de Vries <tdevries@suse.de>
* testsuite/libgomp.c/declare-variant-3-sm30.c: New test.
* testsuite/libgomp.c/declare-variant-3-sm35.c: New test.
* testsuite/libgomp.c/declare-variant-3-sm53.c: New test.
* testsuite/libgomp.c/declare-variant-3-sm70.c: New test.
* testsuite/libgomp.c/declare-variant-3-sm75.c: New test.
* testsuite/libgomp.c/declare-variant-3-sm80.c: New test.
* testsuite/libgomp.c/declare-variant-3.h: New header file.
This was a latent problem, and this commit here now resolves a regression that
after recent commit a78b1ab1df
"amdgcn: Tune default OpenMP/OpenACC GPU utilization" we had (only) seen on a
GCN offloading '-march=gfx908' system:
{+WARNING: program timed out.+}
[-PASS:-]{+FAIL:+} libgomp.oacc-fortran/privatized-ref-2.f90 -DACC_DEVICE_TYPE_radeon=1 -DACC_MEM_SHARED=0 -foffload=amdgcn-amdhsa -O0 execution test
Same for other optimization levels.
Make sure that we're not executing non-parallelized code in gang-redundant
mode, by putting these parts into their own 'parallel' constructs, which then
default to 'num_gangs(1)'.
libgomp/
* testsuite/libgomp.oacc-fortran/privatized-ref-2.f90: Fix OpenACC
gang-redundant execution.
Consider the following omp fragment.
...
#pragma omp target
#pragma omp parallel num_threads (2)
#pragma omp task
;
...
This hangs at -O0 for nvptx.
Investigating the behaviour gives us the following trace of events:
- both threads execute GOMP_task, where they:
- deposit a task, and
- execute gomp_team_barrier_wake
- thread 1 executes gomp_team_barrier_wait_end and, not being the last thread,
proceeds to wait at the team barrier
- thread 0 executes gomp_team_barrier_wait_end and, being the last thread, it
calls gomp_barrier_handle_tasks, where it:
- executes both tasks and marks the team barrier done
- executes a gomp_team_barrier_wake which wakes up thread 1
- thread 1 exits the team barrier
- thread 0 returns from gomp_barrier_handle_tasks and goes to wait at
the team barrier.
- thread 0 hangs.
To understand why there is a hang here, it's good to understand how things
are setup for nvptx. The libgomp/config/nvptx/bar.c implementation is
a copy of the libgomp/config/linux/bar.c implementation, with uses of both
futex_wake and do_wait replaced with uses of ptx insn bar.sync:
...
if (bar->total > 1)
asm ("bar.sync 1, %0;" : : "r" (32 * bar->total));
...
The point where thread 0 goes to wait at the team barrier, corresponds in
the linux implementation with a do_wait. In the linux case, the call to
do_wait doesn't hang, because it's waiting for bar->generation to become
a certain value, and if bar->generation already has that value, it just
proceeds, without any need for coordination with other threads.
In the nvtpx case, the bar.sync waits until thread 1 joins it in the same
logical barrier, which never happens: thread 1 is lingering in the
thread pool at the thread pool barrier (using a different logical barrier),
waiting to join a new team.
The easiest way to fix this is to revert to the posix implementation for
bar.{c,h}. That however falls back on a busy-waiting approach, and
does not take advantage of the ptx bar.sync insn.
Instead, we revert to the linux implementation for bar.c,
and implement bar.c local functions futex_wait and futex_wake using the
bar.sync insn.
The bar.sync insn takes an argument specifying how many threads are
participating, and that doesn't play well with the futex syntax where it's
not clear in advance how many threads will be woken up.
This is solved by waking up all waiting threads each time a futex_wait or
futex_wake happens, and possibly going back to sleep with an updated thread
count.
Tested libgomp on x86_64 with nvptx accelerator.
libgomp/ChangeLog:
2021-04-20 Tom de Vries <tdevries@suse.de>
PR target/99555
* config/nvptx/bar.c (generation_to_barrier): New function, copied
from config/rtems/bar.c.
(futex_wait, futex_wake): New function.
(do_spin, do_wait): New function, copied from config/linux/wait.h.
(gomp_barrier_wait_end, gomp_barrier_wait_last)
(gomp_team_barrier_wake, gomp_team_barrier_wait_end):
(gomp_team_barrier_wait_cancel_end, gomp_team_barrier_cancel): Remove
and replace with include of config/linux/bar.c.
* config/nvptx/bar.h (gomp_barrier_t): Add fields waiters and lock.
(gomp_barrier_init): Init new fields.
* testsuite/libgomp.c-c++-common/task-detach-6.c: Remove nvptx-specific
workarounds.
* testsuite/libgomp.c/pr99555-1.c: Same.
* testsuite/libgomp.fortran/task-detach-6.f90: Same.
When running the libgomp testsuite on x86_64 with nvptx accelerator, we run into:
...
XPASS: libgomp.c/../libgomp.c-c++-common/pr96390.c (test for excess errors)
FAIL: libgomp.c/../libgomp.c-c++-common/pr96390.c execution test
...
The problem is that we're expecting the following ptxas error:
...
XFAIL: libgomp.c/../libgomp.c-c++-common/pr96390.c (test for excess errors)
Excess errors:
ptxas /tmp/ccZYDw8N.o, line 90; error : Call to 'baz' requires call prototype
ptxas /tmp/ccZYDw8N.o, line 90; error : Unknown symbol 'baz'
...
But it's not triggered because ptxas is not in the path, so nvptx-none-as
defaults to --no-verify.
So instead, we run into the same error at execution time.
Fix this by forcing verification using:
...
/* { dg-additional-options "-foffload=-Wa,--verify" \
{ target offload_target_nvptx } } */
...
such that we run into the xfail in this way instead:
...
XFAIL: libgomp.c/../libgomp.c-c++-common/pr96390.c (test for excess errors)
Excess errors:
nvptx-as: error trying to exec 'ptxas': execvp: No such file or directory
nvptx-as: ptxas returned 255 exit status
...
Tested on x86_64-linux with nvptx accelerator.
libgomp/ChangeLog:
2022-02-21 Tom de Vries <tdevries@suse.de>
PR testsuite/104146
* testsuite/libgomp.c++/pr96390.C: Add additional-option
-foffload=-Wa,--verify for nvptx.
* testsuite/libgomp.c-c++-common/pr96390.c: Same.
