Björn Töpel says:
====================
Overview
========
This is the 6th iteration of the series that introduces the BPF
dispatcher, which is a mechanism to avoid indirect calls.
The BPF dispatcher is a multi-way branch code generator, targeted for
BPF programs. E.g. when an XDP program is executed via the
bpf_prog_run_xdp(), it is invoked via an indirect call. With
retpolines enabled, the indirect call has a substantial performance
impact. The dispatcher is a mechanism that transform indirect calls to
direct calls, and therefore avoids the retpoline. The dispatcher is
generated using the BPF JIT, and relies on text poking provided by
bpf_arch_text_poke().
The dispatcher hijacks a trampoline function it via the __fentry__ nop
of the trampoline. One dispatcher instance currently supports up to 48
dispatch points. This can be extended in the future.
In this series, only one dispatcher instance is supported, and the
only user is XDP. The dispatcher is updated when an XDP program is
attached/detached to/from a netdev. An alternative to this could have
been to update the dispatcher at program load point, but as there are
usually more XDP programs loaded than attached, so the latter was
picked.
The XDP dispatcher is always enabled, if available, because it helps
even when retpolines are disabled. Please refer to the "Performance"
section below.
The first patch refactors the image allocation from the BPF trampoline
code. Patch two introduces the dispatcher, and patch three adds a
dispatcher for XDP, and wires up the XDP control-/ fast-path. Patch
four adds the dispatcher to BPF_TEST_RUN. Patch five adds a simple
selftest, and the last adds alignment to jump targets.
I have rebased the series on commit 679152d3a3 ("libbpf: Fix printf
compilation warnings on ppc64le arch").
Generated code, x86-64
======================
The dispatcher currently has a maximum of 48 entries, where one entry
is a unique BPF program. Multiple users of a dispatcher instance using
the same BPF program will share that entry.
The program/slot lookup is performed by a binary search, O(log
n). Let's have a look at the generated code.
The trampoline function has the following signature:
unsigned int tramp(const void *ctx,
const struct bpf_insn *insnsi,
unsigned int (*bpf_func)(const void *,
const struct bpf_insn *))
On Intel x86-64 this means that rdx will contain the bpf_func. To,
make it easier to read, I've let the BPF programs have the following
range: 0xffffffffffffffff (-1) to 0xfffffffffffffff0
(-16). 0xffffffff81c00f10 is the retpoline thunk, in this case
__x86_indirect_thunk_rdx. If retpolines are disabled the thunk will be
a regular indirect call.
The minimal dispatcher will then look like this:
ffffffffc0002000: cmp rdx,0xffffffffffffffff
ffffffffc0002007: je 0xffffffffffffffff ; -1
ffffffffc000200d: jmp 0xffffffff81c00f10
A 16 entry dispatcher looks like this:
ffffffffc0020000: cmp rdx,0xfffffffffffffff7 ; -9
ffffffffc0020007: jg 0xffffffffc0020130
ffffffffc002000d: cmp rdx,0xfffffffffffffff3 ; -13
ffffffffc0020014: jg 0xffffffffc00200a0
ffffffffc002001a: cmp rdx,0xfffffffffffffff1 ; -15
ffffffffc0020021: jg 0xffffffffc0020060
ffffffffc0020023: cmp rdx,0xfffffffffffffff0 ; -16
ffffffffc002002a: jg 0xffffffffc0020040
ffffffffc002002c: cmp rdx,0xfffffffffffffff0 ; -16
ffffffffc0020033: je 0xfffffffffffffff0 ; -16
ffffffffc0020039: jmp 0xffffffff81c00f10
ffffffffc002003e: xchg ax,ax
ffffffffc0020040: cmp rdx,0xfffffffffffffff1 ; -15
ffffffffc0020047: je 0xfffffffffffffff1 ; -15
ffffffffc002004d: jmp 0xffffffff81c00f10
ffffffffc0020052: nop DWORD PTR [rax+rax*1+0x0]
ffffffffc002005a: nop WORD PTR [rax+rax*1+0x0]
ffffffffc0020060: cmp rdx,0xfffffffffffffff2 ; -14
ffffffffc0020067: jg 0xffffffffc0020080
ffffffffc0020069: cmp rdx,0xfffffffffffffff2 ; -14
ffffffffc0020070: je 0xfffffffffffffff2 ; -14
ffffffffc0020076: jmp 0xffffffff81c00f10
ffffffffc002007b: nop DWORD PTR [rax+rax*1+0x0]
ffffffffc0020080: cmp rdx,0xfffffffffffffff3 ; -13
ffffffffc0020087: je 0xfffffffffffffff3 ; -13
ffffffffc002008d: jmp 0xffffffff81c00f10
ffffffffc0020092: nop DWORD PTR [rax+rax*1+0x0]
ffffffffc002009a: nop WORD PTR [rax+rax*1+0x0]
ffffffffc00200a0: cmp rdx,0xfffffffffffffff5 ; -11
ffffffffc00200a7: jg 0xffffffffc00200f0
ffffffffc00200a9: cmp rdx,0xfffffffffffffff4 ; -12
ffffffffc00200b0: jg 0xffffffffc00200d0
ffffffffc00200b2: cmp rdx,0xfffffffffffffff4 ; -12
ffffffffc00200b9: je 0xfffffffffffffff4 ; -12
ffffffffc00200bf: jmp 0xffffffff81c00f10
ffffffffc00200c4: nop DWORD PTR [rax+rax*1+0x0]
ffffffffc00200cc: nop DWORD PTR [rax+0x0]
ffffffffc00200d0: cmp rdx,0xfffffffffffffff5 ; -11
ffffffffc00200d7: je 0xfffffffffffffff5 ; -11
ffffffffc00200dd: jmp 0xffffffff81c00f10
ffffffffc00200e2: nop DWORD PTR [rax+rax*1+0x0]
ffffffffc00200ea: nop WORD PTR [rax+rax*1+0x0]
ffffffffc00200f0: cmp rdx,0xfffffffffffffff6 ; -10
ffffffffc00200f7: jg 0xffffffffc0020110
ffffffffc00200f9: cmp rdx,0xfffffffffffffff6 ; -10
ffffffffc0020100: je 0xfffffffffffffff6 ; -10
ffffffffc0020106: jmp 0xffffffff81c00f10
ffffffffc002010b: nop DWORD PTR [rax+rax*1+0x0]
ffffffffc0020110: cmp rdx,0xfffffffffffffff7 ; -9
ffffffffc0020117: je 0xfffffffffffffff7 ; -9
ffffffffc002011d: jmp 0xffffffff81c00f10
ffffffffc0020122: nop DWORD PTR [rax+rax*1+0x0]
ffffffffc002012a: nop WORD PTR [rax+rax*1+0x0]
ffffffffc0020130: cmp rdx,0xfffffffffffffffb ; -5
ffffffffc0020137: jg 0xffffffffc00201d0
ffffffffc002013d: cmp rdx,0xfffffffffffffff9 ; -7
ffffffffc0020144: jg 0xffffffffc0020190
ffffffffc0020146: cmp rdx,0xfffffffffffffff8 ; -8
ffffffffc002014d: jg 0xffffffffc0020170
ffffffffc002014f: cmp rdx,0xfffffffffffffff8 ; -8
ffffffffc0020156: je 0xfffffffffffffff8 ; -8
ffffffffc002015c: jmp 0xffffffff81c00f10
ffffffffc0020161: nop DWORD PTR [rax+rax*1+0x0]
ffffffffc0020169: nop DWORD PTR [rax+0x0]
ffffffffc0020170: cmp rdx,0xfffffffffffffff9 ; -7
ffffffffc0020177: je 0xfffffffffffffff9 ; -7
ffffffffc002017d: jmp 0xffffffff81c00f10
ffffffffc0020182: nop DWORD PTR [rax+rax*1+0x0]
ffffffffc002018a: nop WORD PTR [rax+rax*1+0x0]
ffffffffc0020190: cmp rdx,0xfffffffffffffffa ; -6
ffffffffc0020197: jg 0xffffffffc00201b0
ffffffffc0020199: cmp rdx,0xfffffffffffffffa ; -6
ffffffffc00201a0: je 0xfffffffffffffffa ; -6
ffffffffc00201a6: jmp 0xffffffff81c00f10
ffffffffc00201ab: nop DWORD PTR [rax+rax*1+0x0]
ffffffffc00201b0: cmp rdx,0xfffffffffffffffb ; -5
ffffffffc00201b7: je 0xfffffffffffffffb ; -5
ffffffffc00201bd: jmp 0xffffffff81c00f10
ffffffffc00201c2: nop DWORD PTR [rax+rax*1+0x0]
ffffffffc00201ca: nop WORD PTR [rax+rax*1+0x0]
ffffffffc00201d0: cmp rdx,0xfffffffffffffffd ; -3
ffffffffc00201d7: jg 0xffffffffc0020220
ffffffffc00201d9: cmp rdx,0xfffffffffffffffc ; -4
ffffffffc00201e0: jg 0xffffffffc0020200
ffffffffc00201e2: cmp rdx,0xfffffffffffffffc ; -4
ffffffffc00201e9: je 0xfffffffffffffffc ; -4
ffffffffc00201ef: jmp 0xffffffff81c00f10
ffffffffc00201f4: nop DWORD PTR [rax+rax*1+0x0]
ffffffffc00201fc: nop DWORD PTR [rax+0x0]
ffffffffc0020200: cmp rdx,0xfffffffffffffffd ; -3
ffffffffc0020207: je 0xfffffffffffffffd ; -3
ffffffffc002020d: jmp 0xffffffff81c00f10
ffffffffc0020212: nop DWORD PTR [rax+rax*1+0x0]
ffffffffc002021a: nop WORD PTR [rax+rax*1+0x0]
ffffffffc0020220: cmp rdx,0xfffffffffffffffe ; -2
ffffffffc0020227: jg 0xffffffffc0020240
ffffffffc0020229: cmp rdx,0xfffffffffffffffe ; -2
ffffffffc0020230: je 0xfffffffffffffffe ; -2
ffffffffc0020236: jmp 0xffffffff81c00f10
ffffffffc002023b: nop DWORD PTR [rax+rax*1+0x0]
ffffffffc0020240: cmp rdx,0xffffffffffffffff ; -1
ffffffffc0020247: je 0xffffffffffffffff ; -1
ffffffffc002024d: jmp 0xffffffff81c00f10
The nops are there to align jump targets to 16 B.
Performance
===========
The tests were performed using the xdp_rxq_info sample program with
the following command-line:
1. XDP_DRV:
# xdp_rxq_info --dev eth0 --action XDP_DROP
2. XDP_SKB:
# xdp_rxq_info --dev eth0 -S --action XDP_DROP
3. xdp-perf, from selftests/bpf:
# test_progs -v -t xdp_perf
Run with mitigations=auto
-------------------------
Baseline:
1. 21.7 Mpps (21736190)
2. 3.8 Mpps (3837582)
3. 15 ns
Dispatcher:
1. 30.2 Mpps (30176320)
2. 4.0 Mpps (4015579)
3. 5 ns
Dispatcher (full; walk all entries, and fallback):
1. 22.0 Mpps (21986704)
2. 3.8 Mpps (3831298)
3. 17 ns
Run with mitigations=off
------------------------
Baseline:
1. 29.9 Mpps (29875135)
2. 4.1 Mpps (4100179)
3. 4 ns
Dispatcher:
1. 30.4 Mpps (30439241)
2. 4.1 Mpps (4109350)
1. 4 ns
Dispatcher (full; walk all entries, and fallback):
1. 28.9 Mpps (28903269)
2. 4.1 Mpps (4080078)
3. 5 ns
xdp-perf runs, aliged vs non-aligned jump targets
-------------------------------------------------
In this test dispatchers of different sizes, with and without jump
target alignment, were exercised. As outlined above the function
lookup is performed via binary search. This means that depending on
the pointer value of the function, it can reside in the upper or lower
part of the search table. The performed tests were:
1. aligned, mititations=auto, function entry < other entries
2. aligned, mititations=auto, function entry > other entries
3. non-aligned, mititations=auto, function entry < other entries
4. non-aligned, mititations=auto, function entry > other entries
5. aligned, mititations=off, function entry < other entries
6. aligned, mititations=off, function entry > other entries
7. non-aligned, mititations=off, function entry < other entries
8. non-aligned, mititations=off, function entry > other entries
The micro benchmarks showed that alignment of jump target has some
positive impact.
A reply to this cover letter will contain complete data for all runs.
Multiple xdp-perf baseline with mitigations=auto
------------------------------------------------
Performance counter stats for './test_progs -v -t xdp_perf' (1024 runs):
16.69 msec task-clock # 0.984 CPUs utilized ( +- 0.08% )
2 context-switches # 0.123 K/sec ( +- 1.11% )
0 cpu-migrations # 0.000 K/sec ( +- 70.68% )
97 page-faults # 0.006 M/sec ( +- 0.05% )
49,254,635 cycles # 2.951 GHz ( +- 0.09% ) (12.28%)
42,138,558 instructions # 0.86 insn per cycle ( +- 0.02% ) (36.15%)
7,315,291 branches # 438.300 M/sec ( +- 0.01% ) (59.43%)
1,011,201 branch-misses # 13.82% of all branches ( +- 0.01% ) (83.31%)
15,440,788 L1-dcache-loads # 925.143 M/sec ( +- 0.00% ) (99.40%)
39,067 L1-dcache-load-misses # 0.25% of all L1-dcache hits ( +- 0.04% )
6,531 LLC-loads # 0.391 M/sec ( +- 0.05% )
442 LLC-load-misses # 6.76% of all LL-cache hits ( +- 0.77% )
<not supported> L1-icache-loads
57,964 L1-icache-load-misses ( +- 0.06% )
15,442,496 dTLB-loads # 925.246 M/sec ( +- 0.00% )
514 dTLB-load-misses # 0.00% of all dTLB cache hits ( +- 0.73% ) (40.57%)
130 iTLB-loads # 0.008 M/sec ( +- 2.75% ) (16.69%)
<not counted> iTLB-load-misses ( +- 8.71% ) (0.60%)
<not supported> L1-dcache-prefetches
<not supported> L1-dcache-prefetch-misses
0.0169558 +- 0.0000127 seconds time elapsed ( +- 0.07% )
Multiple xdp-perf dispatcher with mitigations=auto
--------------------------------------------------
Note that this includes generating the dispatcher.
Performance counter stats for './test_progs -v -t xdp_perf' (1024 runs):
4.80 msec task-clock # 0.953 CPUs utilized ( +- 0.06% )
1 context-switches # 0.258 K/sec ( +- 1.57% )
0 cpu-migrations # 0.000 K/sec
97 page-faults # 0.020 M/sec ( +- 0.05% )
14,185,861 cycles # 2.955 GHz ( +- 0.17% ) (50.49%)
45,691,935 instructions # 3.22 insn per cycle ( +- 0.01% ) (99.19%)
8,346,008 branches # 1738.709 M/sec ( +- 0.00% )
13,046 branch-misses # 0.16% of all branches ( +- 0.10% )
15,443,735 L1-dcache-loads # 3217.365 M/sec ( +- 0.00% )
39,585 L1-dcache-load-misses # 0.26% of all L1-dcache hits ( +- 0.05% )
7,138 LLC-loads # 1.487 M/sec ( +- 0.06% )
671 LLC-load-misses # 9.40% of all LL-cache hits ( +- 0.73% )
<not supported> L1-icache-loads
56,213 L1-icache-load-misses ( +- 0.08% )
15,443,735 dTLB-loads # 3217.365 M/sec ( +- 0.00% )
<not counted> dTLB-load-misses (0.00%)
<not counted> iTLB-loads (0.00%)
<not counted> iTLB-load-misses (0.00%)
<not supported> L1-dcache-prefetches
<not supported> L1-dcache-prefetch-misses
0.00503705 +- 0.00000546 seconds time elapsed ( +- 0.11% )
Revisions
=========
v4->v5: [1]
* Fixed s/xdp_ctx/ctx/ type-o (Toke)
* Marked dispatcher trampoline with noinline attribute (Alexei)
v3->v4: [2]
* Moved away from doing dispatcher lookup based on the trampoline
function, to a model where the dispatcher instance is explicitly
passed to the bpf_dispatcher_change_prog() (Alexei)
v2->v3: [3]
* Removed xdp_call, and instead make the dispatcher available to all
XDP users via bpf_prog_run_xdp() and dev_xdp_install(). (Toke)
* Always enable the dispatcher, if available (Alexei)
* Reuse BPF trampoline image allocator (Alexei)
* Make sure the dispatcher is exercised in selftests (Alexei)
* Only allow one dispatcher, and wire it to XDP
v1->v2: [4]
* Fixed i386 build warning (kbuild robot)
* Made bpf_dispatcher_lookup() static (kbuild robot)
* Make sure xdp_call.h is only enabled for builtins
* Add xdp_call() to ixgbe, mlx4, and mlx5
RFC->v1: [5]
* Improved error handling (Edward and Andrii)
* Explicit cleanup (Andrii)
* Use 32B with sext cmp (Alexei)
* Align jump targets to 16B (Alexei)
* 4 to 16 entries (Toke)
* Added stats to xdp_call_run()
[1] https://lore.kernel.org/bpf/20191211123017.13212-1-bjorn.topel@gmail.com/
[2] https://lore.kernel.org/bpf/20191209135522.16576-1-bjorn.topel@gmail.com/
[3] https://lore.kernel.org/bpf/20191123071226.6501-1-bjorn.topel@gmail.com/
[4] https://lore.kernel.org/bpf/20191119160757.27714-1-bjorn.topel@gmail.com/
[5] https://lore.kernel.org/bpf/20191113204737.31623-1-bjorn.topel@gmail.com/
====================
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
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In order to build the documentation, use ``make htmldocs`` or
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There are various text files in the Documentation/ subdirectory,
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Please read the Documentation/process/changes.rst file, as it contains the
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