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rcu: Discard separate per-CPU callback counts

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Back when there were multiple flavors of RCU, it was necessary to
separately count lazy and non-lazy callbacks for each CPU.  These counts
were used in CONFIG_RCU_FAST_NO_HZ kernels to determine how long a newly
idle CPU should be allowed to sleep before handling its RCU callbacks.
But now that there is only one flavor, the callback counts for a given
CPU's sole rcu_data structure are the counts for that CPU.

This commit therefore removes the rcu_data structure's ->nonlazy_posted
and ->nonlazy_posted_snap fields, the rcu_idle_count_callbacks_posted()
and rcu_cpu_has_callbacks() functions, repurposes the rcu_data structure's
->all_lazy field to record the laziness state at the beginning of the
latest idle sojourn, and modifies CONFIG_RCU_FAST_NO_HZ RCU CPU stall
warnings accordingly.

Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
This commit is contained in:
Paul E. McKenney 2018-11-29 13:28:49 -08:00
parent 8923072664
commit 260e1e4fd8
4 changed files with 21 additions and 75 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

15
Documentation/RCU/stallwarn.txt
View File

@ -219,17 +219,18 @@ an estimate of the total number of RCU callbacks queued across all CPUs
In kernels with CONFIG_RCU_FAST_NO_HZ, more information is printed
for each CPU:
0: (64628 ticks this GP) idle=dd5/3fffffffffffffff/0 softirq=82/543 last_accelerate: a345/d342 nonlazy_posted: 25 .D
0: (64628 ticks this GP) idle=dd5/3fffffffffffffff/0 softirq=82/543 last_accelerate: a345/d342 Nonlazy posted: ..D
The "last_accelerate:" prints the low-order 16 bits (in hex) of the
jiffies counter when this CPU last invoked rcu_try_advance_all_cbs()
from rcu_needs_cpu() or last invoked rcu_accelerate_cbs() from
rcu_prepare_for_idle(). The "nonlazy_posted:" prints the number
of non-lazy callbacks posted since the last call to rcu_needs_cpu().
Finally, an "L" indicates that there are currently no non-lazy callbacks
("." is printed otherwise, as shown above) and "D" indicates that
dyntick-idle processing is enabled ("." is printed otherwise, for example,
if disabled via the "nohz=" kernel boot parameter).
rcu_prepare_for_idle(). The "Nonlazy posted:" indicates lazy-callback
status, so that an "l" indicates that all callbacks were lazy at the start
of the last idle period and an "L" indicates that there are currently
no non-lazy callbacks (in both cases, "." is printed otherwise, as
shown above) and "D" indicates that dyntick-idle processing is enabled
("." is printed otherwise, for example, if disabled via the "nohz="
kernel boot parameter).
If the grace period ends just as the stall warning starts printing,
there will be a spurious stall-warning message, which will include

25
kernel/rcu/tree.c
View File

@ -2878,9 +2878,6 @@ __call_rcu(struct rcu_head *head, rcu_callback_t func, int cpu, bool lazy)
rcu_segcblist_init(&rdp->cblist);
}
rcu_segcblist_enqueue(&rdp->cblist, head, lazy);
if (!lazy)
rcu_idle_count_callbacks_posted();
if (__is_kfree_rcu_offset((unsigned long)func))
trace_rcu_kfree_callback(rcu_state.name, head,
(unsigned long)func,
@ -3110,28 +3107,6 @@ static int rcu_pending(void)
return 0;
}
/*
* Return true if the specified CPU has any callback. If all_lazy is
* non-NULL, store an indication of whether all callbacks are lazy.
* (If there are no callbacks, all of them are deemed to be lazy.)
*/
static bool rcu_cpu_has_callbacks(bool *all_lazy)
{
bool al = true;
bool hc = false;
struct rcu_data *rdp;
rdp = this_cpu_ptr(&rcu_data);
if (!rcu_segcblist_empty(&rdp->cblist)) {
hc = true;
if (rcu_segcblist_n_nonlazy_cbs(&rdp->cblist))
al = false;
}
if (all_lazy)
*all_lazy = al;
return hc;
}
/*
* Helper function for rcu_barrier() tracing. If tracing is disabled,
* the compiler is expected to optimize this away.

6
kernel/rcu/tree.h
View File

@ -193,10 +193,7 @@ struct rcu_data {
bool rcu_need_heavy_qs; /* GP old, so heavy quiescent state! */
bool rcu_urgent_qs; /* GP old need light quiescent state. */
#ifdef CONFIG_RCU_FAST_NO_HZ
bool all_lazy; /* Are all CPU's CBs lazy? */
unsigned long nonlazy_posted; /* # times non-lazy CB posted to CPU. */
unsigned long nonlazy_posted_snap;
/* Nonlazy_posted snapshot. */
bool all_lazy; /* All CPU's CBs lazy at idle start? */
unsigned long last_accelerate; /* Last jiffy CBs were accelerated. */
unsigned long last_advance_all; /* Last jiffy CBs were all advanced. */
int tick_nohz_enabled_snap; /* Previously seen value from sysfs. */
@ -430,7 +427,6 @@ static void __init rcu_spawn_boost_kthreads(void);
static void rcu_prepare_kthreads(int cpu);
static void rcu_cleanup_after_idle(void);
static void rcu_prepare_for_idle(void);
static void rcu_idle_count_callbacks_posted(void);
static bool rcu_preempt_has_tasks(struct rcu_node *rnp);
static bool rcu_preempt_need_deferred_qs(struct task_struct *t);
static void rcu_preempt_deferred_qs(struct task_struct *t);

50
kernel/rcu/tree_plugin.h
View File

@ -1474,7 +1474,7 @@ static void rcu_prepare_kthreads(int cpu)
int rcu_needs_cpu(u64 basemono, u64 *nextevt)
{
*nextevt = KTIME_MAX;
return rcu_cpu_has_callbacks(NULL);
return !rcu_segcblist_empty(&this_cpu_ptr(&rcu_data)->cblist);
}
/*
@ -1493,14 +1493,6 @@ static void rcu_prepare_for_idle(void)
{
}
/*
* Don't bother keeping a running count of the number of RCU callbacks
* posted because CONFIG_RCU_FAST_NO_HZ=n.
*/
static void rcu_idle_count_callbacks_posted(void)
{
}
#else /* #if !defined(CONFIG_RCU_FAST_NO_HZ) */
/*
@ -1583,11 +1575,8 @@ int rcu_needs_cpu(u64 basemono, u64 *nextevt)
lockdep_assert_irqs_disabled();
/* Snapshot to detect later posting of non-lazy callback. */
rdp->nonlazy_posted_snap = rdp->nonlazy_posted;
/* If no callbacks, RCU doesn't need the CPU. */
if (!rcu_cpu_has_callbacks(&rdp->all_lazy)) {
if (rcu_segcblist_empty(&rdp->cblist)) {
*nextevt = KTIME_MAX;
return 0;
}
@ -1601,11 +1590,12 @@ int rcu_needs_cpu(u64 basemono, u64 *nextevt)
rdp->last_accelerate = jiffies;
/* Request timer delay depending on laziness, and round. */
if (!rdp->all_lazy) {
rdp->all_lazy = !rcu_segcblist_n_nonlazy_cbs(&rdp->cblist);
if (rdp->all_lazy) {
dj = round_jiffies(rcu_idle_lazy_gp_delay + jiffies) - jiffies;
} else {
dj = round_up(rcu_idle_gp_delay + jiffies,
rcu_idle_gp_delay) - jiffies;
} else {
dj = round_jiffies(rcu_idle_lazy_gp_delay + jiffies) - jiffies;
}
*nextevt = basemono + dj * TICK_NSEC;
return 0;
@ -1635,7 +1625,7 @@ static void rcu_prepare_for_idle(void)
/* Handle nohz enablement switches conservatively. */
tne = READ_ONCE(tick_nohz_active);
if (tne != rdp->tick_nohz_enabled_snap) {
if (rcu_cpu_has_callbacks(NULL))
if (!rcu_segcblist_empty(&rdp->cblist))
invoke_rcu_core(); /* force nohz to see update. */
rdp->tick_nohz_enabled_snap = tne;
return;
@ -1648,10 +1638,8 @@ static void rcu_prepare_for_idle(void)
* callbacks, invoke RCU core for the side-effect of recalculating
* idle duration on re-entry to idle.
*/
if (rdp->all_lazy &&
rdp->nonlazy_posted != rdp->nonlazy_posted_snap) {
if (rdp->all_lazy && rcu_segcblist_n_nonlazy_cbs(&rdp->cblist)) {
rdp->all_lazy = false;
rdp->nonlazy_posted_snap = rdp->nonlazy_posted;
invoke_rcu_core();
return;
}
@ -1687,19 +1675,6 @@ static void rcu_cleanup_after_idle(void)
invoke_rcu_core();
}
/*
* Keep a running count of the number of non-lazy callbacks posted
* on this CPU. This running counter (which is never decremented) allows
* rcu_prepare_for_idle() to detect when something out of the idle loop
* posts a callback, even if an equal number of callbacks are invoked.
* Of course, callbacks should only be posted from within a trace event
* designed to be called from idle or from within RCU_NONIDLE().
*/
static void rcu_idle_count_callbacks_posted(void)
{
__this_cpu_add(rcu_data.nonlazy_posted, 1);
}
#endif /* #else #if !defined(CONFIG_RCU_FAST_NO_HZ) */
#ifdef CONFIG_RCU_FAST_NO_HZ
@ -1707,13 +1682,12 @@ static void rcu_idle_count_callbacks_posted(void)
static void print_cpu_stall_fast_no_hz(char *cp, int cpu)
{
struct rcu_data *rdp = &per_cpu(rcu_data, cpu);
unsigned long nlpd = rdp->nonlazy_posted - rdp->nonlazy_posted_snap;
sprintf(cp, "last_accelerate: %04lx/%04lx, nonlazy_posted: %ld, %c%c",
sprintf(cp, "last_accelerate: %04lx/%04lx, Nonlazy posted: %c%c%c",
rdp->last_accelerate & 0xffff, jiffies & 0xffff,
ulong2long(nlpd),
rdp->all_lazy ? 'L' : '.',
rdp->tick_nohz_enabled_snap ? '.' : 'D');
".l"[rdp->all_lazy],
".L"[!rcu_segcblist_n_nonlazy_cbs(&rdp->cblist)],
".D"[!rdp->tick_nohz_enabled_snap]);
}
#else /* #ifdef CONFIG_RCU_FAST_NO_HZ */