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Date: Wed, 19 Jun 2024 12:15:04 +0100
From: Qais Yousef <qyousef@...alina.io>
To: John Stultz <jstultz@...gle.com>
Cc: LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...hat.com>,
Peter Zijlstra <peterz@...radead.org>,
Juri Lelli <juri.lelli@...hat.com>,
Vincent Guittot <vincent.guittot@...aro.org>,
Dietmar Eggemann <dietmar.eggemann@....com>,
Steven Rostedt <rostedt@...dmis.org>,
Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
Daniel Bristot de Oliveira <bristot@...hat.com>,
Valentin Schneider <vschneid@...hat.com>,
Johannes Weiner <hannes@...xchg.org>,
Suren Baghdasaryan <surenb@...gle.com>,
Chengming Zhou <zhouchengming@...edance.com>,
Thomas Gleixner <tglx@...utronix.de>,
Frederic Weisbecker <frederic@...nel.org>,
Joel Fernandes <joel@...lfernandes.org>, kernel-team@...roid.com,
Jimmy Shiu <jimmyshiu@...gle.com>
Subject: Re: [PATCH] sched: Move psi_account_irqtime() out of
update_rq_clock_task() hotpath
On 06/18/24 14:58, John Stultz wrote:
> It was reported that in moving to 6.1, a larger then 10%
> regression was seen in the performance of
> clock_gettime(CLOCK_THREAD_CPUTIME_ID,...).
>
> Using a simple reproducer, I found:
> 5.10:
> 100000000 calls in 24345994193 ns => 243.460 ns per call
> 100000000 calls in 24288172050 ns => 242.882 ns per call
> 100000000 calls in 24289135225 ns => 242.891 ns per call
>
> 6.1:
> 100000000 calls in 28248646742 ns => 282.486 ns per call
> 100000000 calls in 28227055067 ns => 282.271 ns per call
> 100000000 calls in 28177471287 ns => 281.775 ns per call
>
> The cause of this was finally narrowed down to the addition of
> psi_account_irqtime() in update_rq_clock_task(), in commit
> 52b1364ba0b1 ("sched/psi: Add PSI_IRQ to track IRQ/SOFTIRQ
> pressure").
>
> In my initial attempt to resolve this, I leaned towards moving
> all accounting work out of the clock_gettime() call path, but it
> wasn't very pretty, so it will have to wait for a later deeper
> rework. Instead, Peter shared this approach:
>
> Rework psi_account_irqtime() to use its own psi_irq_time base
> for accounting, and move it out of the hotpath, calling it
> instead from sched_tick() and __schedule().
I think we can be more accurate towards group change if we check in
sched_move_task() if task is running. But as-is doesn't change current behavior
AFAICT.
Reviewed-by: Qais Yousef <qyousef@...alina.io>
>
> In testing this, we found the importance of ensuring
> psi_account_irqtime() is run under the rq_lock, which Johannes
> Weiner helpfully explained, so also add some lockdep annotations
> to make that requirement clear.
>
> With this change the performance is back in-line with 5.10:
> 6.1+fix:
> 100000000 calls in 24297324597 ns => 242.973 ns per call
> 100000000 calls in 24318869234 ns => 243.189 ns per call
> 100000000 calls in 24291564588 ns => 242.916 ns per call
>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Juri Lelli <juri.lelli@...hat.com>
> Cc: Vincent Guittot <vincent.guittot@...aro.org>
> Cc: Dietmar Eggemann <dietmar.eggemann@....com>
> Cc: Steven Rostedt <rostedt@...dmis.org>
> Cc: Ben Segall <bsegall@...gle.com>
> Cc: Mel Gorman <mgorman@...e.de>
> Cc: Daniel Bristot de Oliveira <bristot@...hat.com>
> Cc: Valentin Schneider <vschneid@...hat.com>
> Cc: Johannes Weiner <hannes@...xchg.org>
> Cc: Suren Baghdasaryan <surenb@...gle.com>
> Cc: Chengming Zhou <zhouchengming@...edance.com>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Frederic Weisbecker <frederic@...nel.org>
> Cc: Qais Yousef <qyousef@...alina.io>
> Cc: Joel Fernandes <joel@...lfernandes.org>
> Cc: kernel-team@...roid.com
> Originally-by: Peter Zijlstra <peterz@...radead.org>
> Reported-by: Jimmy Shiu <jimmyshiu@...gle.com>
> Signed-off-by: John Stultz <jstultz@...gle.com>
> ---
> kernel/sched/core.c | 7 +++++--
> kernel/sched/psi.c | 21 ++++++++++++++++-----
> kernel/sched/sched.h | 1 +
> kernel/sched/stats.h | 11 ++++++++---
> 4 files changed, 30 insertions(+), 10 deletions(-)
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index bcf2c4cc0522..59ce0841eb1f 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -723,7 +723,6 @@ static void update_rq_clock_task(struct rq *rq, s64 delta)
>
> rq->prev_irq_time += irq_delta;
> delta -= irq_delta;
> - psi_account_irqtime(rq->curr, irq_delta);
> delayacct_irq(rq->curr, irq_delta);
> #endif
> #ifdef CONFIG_PARAVIRT_TIME_ACCOUNTING
> @@ -5665,7 +5664,7 @@ void sched_tick(void)
> {
> int cpu = smp_processor_id();
> struct rq *rq = cpu_rq(cpu);
> - struct task_struct *curr = rq->curr;
> + struct task_struct *curr;
> struct rq_flags rf;
> unsigned long hw_pressure;
> u64 resched_latency;
> @@ -5677,6 +5676,9 @@ void sched_tick(void)
>
> rq_lock(rq, &rf);
>
> + curr = rq->curr;
> + psi_account_irqtime(rq, curr, NULL);
> +
> update_rq_clock(rq);
> hw_pressure = arch_scale_hw_pressure(cpu_of(rq));
> update_hw_load_avg(rq_clock_task(rq), rq, hw_pressure);
> @@ -6737,6 +6739,7 @@ static void __sched notrace __schedule(unsigned int sched_mode)
> ++*switch_count;
>
> migrate_disable_switch(rq, prev);
> + psi_account_irqtime(rq, prev, next);
> psi_sched_switch(prev, next, !task_on_rq_queued(prev));
>
> trace_sched_switch(sched_mode & SM_MASK_PREEMPT, prev, next, prev_state);
> diff --git a/kernel/sched/psi.c b/kernel/sched/psi.c
> index 7b4aa5809c0f..507d7b8d79af 100644
> --- a/kernel/sched/psi.c
> +++ b/kernel/sched/psi.c
> @@ -773,6 +773,7 @@ static void psi_group_change(struct psi_group *group, int cpu,
> enum psi_states s;
> u32 state_mask;
>
> + lockdep_assert_rq_held(cpu_rq(cpu));
> groupc = per_cpu_ptr(group->pcpu, cpu);
>
> /*
> @@ -991,22 +992,32 @@ void psi_task_switch(struct task_struct *prev, struct task_struct *next,
> }
>
> #ifdef CONFIG_IRQ_TIME_ACCOUNTING
> -void psi_account_irqtime(struct task_struct *task, u32 delta)
> +void psi_account_irqtime(struct rq *rq, struct task_struct *curr, struct task_struct *prev)
> {
> - int cpu = task_cpu(task);
> + int cpu = task_cpu(curr);
> struct psi_group *group;
> struct psi_group_cpu *groupc;
> - u64 now;
> + u64 now, irq;
> + s64 delta;
>
> if (static_branch_likely(&psi_disabled))
> return;
>
> - if (!task->pid)
> + if (!curr->pid)
> + return;
> +
> + lockdep_assert_rq_held(rq);
> + group = task_psi_group(curr);
> + if (prev && task_psi_group(prev) == group)
> return;
>
> now = cpu_clock(cpu);
> + irq = irq_time_read(cpu);
> + delta = (s64)(irq - rq->psi_irq_time);
> + if (delta < 0)
> + return;
> + rq->psi_irq_time = irq;
>
> - group = task_psi_group(task);
> do {
> if (!group->enabled)
> continue;
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index a831af102070..ef20c61004eb 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -1126,6 +1126,7 @@ struct rq {
>
> #ifdef CONFIG_IRQ_TIME_ACCOUNTING
> u64 prev_irq_time;
> + u64 psi_irq_time;
> #endif
> #ifdef CONFIG_PARAVIRT
> u64 prev_steal_time;
> diff --git a/kernel/sched/stats.h b/kernel/sched/stats.h
> index 38f3698f5e5b..b02dfc322951 100644
> --- a/kernel/sched/stats.h
> +++ b/kernel/sched/stats.h
> @@ -110,8 +110,12 @@ __schedstats_from_se(struct sched_entity *se)
> void psi_task_change(struct task_struct *task, int clear, int set);
> void psi_task_switch(struct task_struct *prev, struct task_struct *next,
> bool sleep);
> -void psi_account_irqtime(struct task_struct *task, u32 delta);
> -
> +#ifdef CONFIG_IRQ_TIME_ACCOUNTING
> +void psi_account_irqtime(struct rq *rq, struct task_struct *curr, struct task_struct *prev);
> +#else
> +static inline void psi_account_irqtime(struct rq *rq, struct task_struct *curr,
> + struct task_struct *prev) {}
> +#endif /*CONFIG_IRQ_TIME_ACCOUNTING */
> /*
> * PSI tracks state that persists across sleeps, such as iowaits and
> * memory stalls. As a result, it has to distinguish between sleeps,
> @@ -192,7 +196,8 @@ static inline void psi_ttwu_dequeue(struct task_struct *p) {}
> static inline void psi_sched_switch(struct task_struct *prev,
> struct task_struct *next,
> bool sleep) {}
> -static inline void psi_account_irqtime(struct task_struct *task, u32 delta) {}
> +static inline void psi_account_irqtime(struct rq *rq, struct task_struct *curr,
> + struct task_struct *prev) {}
> #endif /* CONFIG_PSI */
>
> #ifdef CONFIG_SCHED_INFO
> --
> 2.45.2.627.g7a2c4fd464-goog
>
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