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Thu, 5 Jan 2023 09:54:47 -0300 (-03) Message-ID: <20230105125248.932725463@redhat.com> User-Agent: quilt/0.66 Date: Thu, 05 Jan 2023 09:52:23 -0300 From: Marcelo Tosatti To: atomlin@atomlin.com, frederic@kernel.org Cc: cl@linux.com, tglx@linutronix.de, mingo@kernel.org, peterz@infradead.org, pauld@redhat.com, neelx@redhat.com, oleksandr@natalenko.name, linux-kernel@vger.kernel.org, linux-mm@kvack.org, Marcelo Tosatti Subject: [PATCH v13 5/6] tick/sched: Ensure quiet_vmstat() is called when the idle tick was stopped too References: <20230105125218.031928326@redhat.com> MIME-Version: 1.0 X-Scanned-By: MIMEDefang 3.1 on 10.11.54.2 X-Rspam-User: X-Rspamd-Queue-Id: 06EA814000E X-Rspamd-Server: rspam01 X-Stat-Signature: i9igoff74f9qkji1hjszimiow1eufwjk X-HE-Tag: 1672923404-600566 X-HE-Meta: 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: From: Aaron Tomlin In the context of the idle task and an adaptive-tick mode/or a nohz_full CPU, quiet_vmstat() can be called: before stopping the idle tick, entering an idle state and on exit. In particular, for the latter case, when the idle task is required to reschedule, the idle tick can remain stopped and the timer expiration time endless i.e., KTIME_MAX. Now, indeed before a nohz_full CPU enters an idle state, CPU-specific vmstat counters should be processed to ensure the respective values have been reset and folded into the zone specific 'vm_stat[]'. That being said, it can only occur when: the idle tick was previously stopped, and reprogramming of the timer is not required. A customer provided some evidence which indicates that the idle tick was stopped; albeit, CPU-specific vmstat counters still remained populated. Thus one can only assume quiet_vmstat() was not invoked on return to the idle loop. If I understand correctly, I suspect this divergence might erroneously prevent a reclaim attempt by kswapd. If the number of zone specific free pages are below their per-cpu drift value then zone_page_state_snapshot() is used to compute a more accurate view of the aforementioned statistic. Thus any task blocked on the NUMA node specific pfmemalloc_wait queue will be unable to make significant progress via direct reclaim unless it is killed after being woken up by kswapd (see throttle_direct_reclaim()). Consider the following theoretical scenario: - Note: CPU X is part of 'tick_nohz_full_mask' 1. CPU Y migrated running task A to CPU X that was in an idle state i.e. waiting for an IRQ; marked the current task on CPU X to need/or require a reschedule i.e., set TIF_NEED_RESCHED and invoked a reschedule IPI to CPU X (see sched_move_task()) 2. CPU X acknowledged the reschedule IPI. Generic idle loop code noticed the TIF_NEED_RESCHED flag against the idle task and attempts to exit of the loop and calls the main scheduler function i.e. __schedule(). Since the idle tick was previously stopped no scheduling-clock tick would occur. So, no deferred timers would be handled 3. Post transition to kernel execution Task A running on CPU X, indirectly released a few pages (e.g. see __free_one_page()); CPU X's 'vm_stat_diff[NR_FREE_PAGES]' was updated and zone specific 'vm_stat[]' update was deferred as per the CPU-specific stat threshold 4. Task A does invoke exit(2) and the kernel does remove the task from the run-queue; the idle task was selected to execute next since there are no other runnable tasks assigned to the given CPU (see pick_next_task() and pick_next_task_idle()) 5. On return to the idle loop since the idle tick was already stopped and can remain so (see [1] below) e.g. no pending soft IRQs, no attempt is made to zero and fold CPU X's vmstat counters since reprogramming of the scheduling-clock tick is not required/or needed (see [2]) ... do_idle { __current_set_polling() tick_nohz_idle_enter() while (!need_resched()) { local_irq_disable() ... /* No polling or broadcast event */ cpuidle_idle_call() { if (cpuidle_not_available(drv, dev)) { tick_nohz_idle_stop_tick() __tick_nohz_idle_stop_tick(this_cpu_ptr(&tick_cpu_sched)) { int cpu = smp_processor_id() if (ts->timer_expires_base) expires = ts->timer_expires else if (can_stop_idle_tick(cpu, ts)) (1) -------> expires = tick_nohz_next_event(ts, cpu) else return ts->idle_calls++ if (expires > 0LL) { tick_nohz_stop_tick(ts, cpu) { if (ts->tick_stopped && (expires == ts->next_tick)) { (2) -------> if (tick == KTIME_MAX || ts->next_tick == hrtimer_get_expires(&ts->sched_timer)) return } ... } So, the idea of this patch is to ensure refresh_cpu_vm_stats(false) is called, when it is appropriate, on return to the idle loop if the idle tick was previously stopped too. A trivial test program was used to determine the impact of the proposed changes and under vanilla. The nanosleep(2) system call was used several times to suspend execution for a period of time to approximately compute the number of CPU-cycles in the idle code path. The following is an average count of CPU-cycles: Vanilla Modified Cycles per idle loop 151858 153258 (+1.0%) Signed-off-by: Aaron Tomlin Signed-off-by: Marcelo Tosatti --- kernel/time/tick-sched.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) Index: linux-2.6/kernel/time/tick-sched.c =================================================================== --- linux-2.6.orig/kernel/time/tick-sched.c +++ linux-2.6/kernel/time/tick-sched.c @@ -929,13 +929,14 @@ static void tick_nohz_stop_tick(struct t */ if (!ts->tick_stopped) { calc_load_nohz_start(); - quiet_vmstat(false); ts->last_tick = hrtimer_get_expires(&ts->sched_timer); ts->tick_stopped = 1; trace_tick_stop(1, TICK_DEP_MASK_NONE); } + /* Attempt to fold when the idle tick is stopped or not */ + quiet_vmstat(false); ts->next_tick = tick; /*