commit ddae0ca2a8fe12d0e24ab10ba759c3fbd755ada8 upstream.

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().

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

Reported-by: Jimmy Shiu jimmyshiu@google.com Originally-by: Peter Zijlstra peterz@infradead.org Signed-off-by: John Stultz jstultz@google.com Signed-off-by: Peter Zijlstra (Intel) peterz@infradead.org Reviewed-by: Chengming Zhou chengming.zhou@linux.dev Reviewed-by: Qais Yousef qyousef@layalina.io Link: https://lore.kernel.org/r/20240618215909.4099720-1-jstultz@google.com Fixes: 52b1364ba0b1 ("sched/psi: Add PSI_IRQ to track IRQ/SOFTIRQ pressure") [jstultz: Fixed up minor collisions w/ 6.1-stable] Signed-off-by: John Stultz jstultz@google.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 d71234729edb..cac41c49bd2f 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -701,7 +701,6 @@ 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); #endif #ifdef CONFIG_PARAVIRT_TIME_ACCOUNTING if (static_key_false((&paravirt_steal_rq_enabled))) { @@ -5500,7 +5499,7 @@ void scheduler_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 thermal_pressure; u64 resched_latency; @@ -5512,6 +5511,9 @@ void scheduler_tick(void)

rq_lock(rq, &rf);

+ curr = rq->curr; + psi_account_irqtime(rq, curr, NULL); + update_rq_clock(rq); thermal_pressure = arch_scale_thermal_pressure(cpu_of(rq)); update_thermal_load_avg(rq_clock_thermal(rq), rq, thermal_pressure); @@ -6550,6 +6552,7 @@ 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 80d8c10e9363..81dbced92df5 100644 --- a/kernel/sched/psi.c +++ b/kernel/sched/psi.c @@ -785,6 +785,7 @@ 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);

/* @@ -1003,19 +1004,29 @@ 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 (!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 b62d53d7c264..81d9698f0a1e 100644 --- a/kernel/sched/sched.h +++ b/kernel/sched/sched.h @@ -1084,6 +1084,7 @@ 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 84a188913cc9..b49a96fad1d2 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, @@ -206,7 +210,8 @@ 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