6.15-stable review patch. If anyone has any objections, please let me know.
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From: Peter Zijlstra peterz@infradead.org
[ Upstream commit 570c8efd5eb79c3725ba439ce105ed1bedc5acd9 ]
Dietmar reported that commit 3840cbe24cf0 ("sched: psi: fix bogus pressure spikes from aggregation race") caused a regression for him on a high context switch rate benchmark (schbench) due to the now repeating cpu_clock() calls.
In particular the problem is that get_recent_times() will extrapolate the current state to 'now'. But if an update uses a timestamp from before the start of the update, it is possible to get two reads with inconsistent results. It is effectively back-dating an update.
(note that this all hard-relies on the clock being synchronized across CPUs -- if this is not the case, all bets are off).
Combine this problem with the fact that there are per-group-per-cpu seqcounts, the commit in question pushed the clock read into the group iteration, causing tree-depth cpu_clock() calls. On architectures where cpu_clock() has appreciable overhead, this hurts.
Instead move to a per-cpu seqcount, which allows us to have a single clock read for all group updates, increasing internal consistency and lowering update overhead. This comes at the cost of a longer update side (proportional to the tree depth) which can cause the read side to retry more often.
Fixes: 3840cbe24cf0 ("sched: psi: fix bogus pressure spikes from aggregation race") Reported-by: Dietmar Eggemann dietmar.eggemann@arm.com Signed-off-by: Peter Zijlstra (Intel) peterz@infradead.org Acked-by: Johannes Weiner hannes@cmpxchg.org Tested-by: Dietmar Eggemann dietmar.eggemann@arm.com, Link: https://lkml.kernel.org/20250522084844.GC31726@noisy.programming.kicks-ass.n... Signed-off-by: Sasha Levin sashal@kernel.org --- include/linux/psi_types.h | 6 +- kernel/sched/psi.c | 121 +++++++++++++++++++++----------------- 2 files changed, 68 insertions(+), 59 deletions(-)
diff --git a/include/linux/psi_types.h b/include/linux/psi_types.h index f1fd3a8044e0..dd10c22299ab 100644 --- a/include/linux/psi_types.h +++ b/include/linux/psi_types.h @@ -84,11 +84,9 @@ enum psi_aggregators { struct psi_group_cpu { /* 1st cacheline updated by the scheduler */
- /* Aggregator needs to know of concurrent changes */ - seqcount_t seq ____cacheline_aligned_in_smp; - /* States of the tasks belonging to this group */ - unsigned int tasks[NR_PSI_TASK_COUNTS]; + unsigned int tasks[NR_PSI_TASK_COUNTS] + ____cacheline_aligned_in_smp;
/* Aggregate pressure state derived from the tasks */ u32 state_mask; diff --git a/kernel/sched/psi.c b/kernel/sched/psi.c index 1396674fa722..c62f4316a2b9 100644 --- a/kernel/sched/psi.c +++ b/kernel/sched/psi.c @@ -172,6 +172,28 @@ struct psi_group psi_system = { .pcpu = &system_group_pcpu, };
+static DEFINE_PER_CPU(seqcount_t, psi_seq); + +static inline void psi_write_begin(int cpu) +{ + write_seqcount_begin(per_cpu_ptr(&psi_seq, cpu)); +} + +static inline void psi_write_end(int cpu) +{ + write_seqcount_end(per_cpu_ptr(&psi_seq, cpu)); +} + +static inline u32 psi_read_begin(int cpu) +{ + return read_seqcount_begin(per_cpu_ptr(&psi_seq, cpu)); +} + +static inline bool psi_read_retry(int cpu, u32 seq) +{ + return read_seqcount_retry(per_cpu_ptr(&psi_seq, cpu), seq); +} + static void psi_avgs_work(struct work_struct *work);
static void poll_timer_fn(struct timer_list *t); @@ -182,7 +204,7 @@ static void group_init(struct psi_group *group)
group->enabled = true; for_each_possible_cpu(cpu) - seqcount_init(&per_cpu_ptr(group->pcpu, cpu)->seq); + seqcount_init(per_cpu_ptr(&psi_seq, cpu)); group->avg_last_update = sched_clock(); group->avg_next_update = group->avg_last_update + psi_period; mutex_init(&group->avgs_lock); @@ -262,14 +284,14 @@ static void get_recent_times(struct psi_group *group, int cpu,
/* Snapshot a coherent view of the CPU state */ do { - seq = read_seqcount_begin(&groupc->seq); + seq = psi_read_begin(cpu); now = cpu_clock(cpu); memcpy(times, groupc->times, sizeof(groupc->times)); state_mask = groupc->state_mask; state_start = groupc->state_start; if (cpu == current_cpu) memcpy(tasks, groupc->tasks, sizeof(groupc->tasks)); - } while (read_seqcount_retry(&groupc->seq, seq)); + } while (psi_read_retry(cpu, seq));
/* Calculate state time deltas against the previous snapshot */ for (s = 0; s < NR_PSI_STATES; s++) { @@ -768,30 +790,20 @@ static void record_times(struct psi_group_cpu *groupc, u64 now) groupc->times[PSI_NONIDLE] += delta; }
+#define for_each_group(iter, group) \ + for (typeof(group) iter = group; iter; iter = iter->parent) + static void psi_group_change(struct psi_group *group, int cpu, unsigned int clear, unsigned int set, - bool wake_clock) + u64 now, bool wake_clock) { struct psi_group_cpu *groupc; unsigned int t, m; u32 state_mask; - u64 now;
lockdep_assert_rq_held(cpu_rq(cpu)); groupc = per_cpu_ptr(group->pcpu, cpu);
- /* - * First we update the task counts according to the state - * change requested through the @clear and @set bits. - * - * Then if the cgroup PSI stats accounting enabled, we - * assess the aggregate resource states this CPU's tasks - * have been in since the last change, and account any - * SOME and FULL time these may have resulted in. - */ - write_seqcount_begin(&groupc->seq); - now = cpu_clock(cpu); - /* * Start with TSK_ONCPU, which doesn't have a corresponding * task count - it's just a boolean flag directly encoded in @@ -843,7 +855,6 @@ static void psi_group_change(struct psi_group *group, int cpu,
groupc->state_mask = state_mask;
- write_seqcount_end(&groupc->seq); return; }
@@ -864,8 +875,6 @@ static void psi_group_change(struct psi_group *group, int cpu,
groupc->state_mask = state_mask;
- write_seqcount_end(&groupc->seq); - if (state_mask & group->rtpoll_states) psi_schedule_rtpoll_work(group, 1, false);
@@ -900,24 +909,29 @@ static void psi_flags_change(struct task_struct *task, int clear, int set) void psi_task_change(struct task_struct *task, int clear, int set) { int cpu = task_cpu(task); - struct psi_group *group; + u64 now;
if (!task->pid) return;
psi_flags_change(task, clear, set);
- group = task_psi_group(task); - do { - psi_group_change(group, cpu, clear, set, true); - } while ((group = group->parent)); + psi_write_begin(cpu); + now = cpu_clock(cpu); + for_each_group(group, task_psi_group(task)) + psi_group_change(group, cpu, clear, set, now, true); + psi_write_end(cpu); }
void psi_task_switch(struct task_struct *prev, struct task_struct *next, bool sleep) { - struct psi_group *group, *common = NULL; + struct psi_group *common = NULL; int cpu = task_cpu(prev); + u64 now; + + psi_write_begin(cpu); + now = cpu_clock(cpu);
if (next->pid) { psi_flags_change(next, 0, TSK_ONCPU); @@ -926,16 +940,15 @@ void psi_task_switch(struct task_struct *prev, struct task_struct *next, * ancestors with @prev, those will already have @prev's * TSK_ONCPU bit set, and we can stop the iteration there. */ - group = task_psi_group(next); - do { - if (per_cpu_ptr(group->pcpu, cpu)->state_mask & - PSI_ONCPU) { + for_each_group(group, task_psi_group(next)) { + struct psi_group_cpu *groupc = per_cpu_ptr(group->pcpu, cpu); + + if (groupc->state_mask & PSI_ONCPU) { common = group; break; } - - psi_group_change(group, cpu, 0, TSK_ONCPU, true); - } while ((group = group->parent)); + psi_group_change(group, cpu, 0, TSK_ONCPU, now, true); + } }
if (prev->pid) { @@ -968,12 +981,11 @@ void psi_task_switch(struct task_struct *prev, struct task_struct *next,
psi_flags_change(prev, clear, set);
- group = task_psi_group(prev); - do { + for_each_group(group, task_psi_group(prev)) { if (group == common) break; - psi_group_change(group, cpu, clear, set, wake_clock); - } while ((group = group->parent)); + psi_group_change(group, cpu, clear, set, now, wake_clock); + }
/* * TSK_ONCPU is handled up to the common ancestor. If there are @@ -983,20 +995,21 @@ void psi_task_switch(struct task_struct *prev, struct task_struct *next, */ if ((prev->psi_flags ^ next->psi_flags) & ~TSK_ONCPU) { clear &= ~TSK_ONCPU; - for (; group; group = group->parent) - psi_group_change(group, cpu, clear, set, wake_clock); + for_each_group(group, common) + psi_group_change(group, cpu, clear, set, now, wake_clock); } } + psi_write_end(cpu); }
#ifdef CONFIG_IRQ_TIME_ACCOUNTING void psi_account_irqtime(struct rq *rq, struct task_struct *curr, struct task_struct *prev) { int cpu = task_cpu(curr); - struct psi_group *group; struct psi_group_cpu *groupc; s64 delta; u64 irq; + u64 now;
if (static_branch_likely(&psi_disabled) || !irqtime_enabled()) return; @@ -1005,8 +1018,7 @@ void psi_account_irqtime(struct rq *rq, struct task_struct *curr, struct task_st return;
lockdep_assert_rq_held(rq); - group = task_psi_group(curr); - if (prev && task_psi_group(prev) == group) + if (prev && task_psi_group(prev) == task_psi_group(curr)) return;
irq = irq_time_read(cpu); @@ -1015,25 +1027,22 @@ void psi_account_irqtime(struct rq *rq, struct task_struct *curr, struct task_st return; rq->psi_irq_time = irq;
- do { - u64 now; + psi_write_begin(cpu); + now = cpu_clock(cpu);
+ for_each_group(group, task_psi_group(curr)) { if (!group->enabled) continue;
groupc = per_cpu_ptr(group->pcpu, cpu);
- write_seqcount_begin(&groupc->seq); - now = cpu_clock(cpu); - record_times(groupc, now); groupc->times[PSI_IRQ_FULL] += delta;
- write_seqcount_end(&groupc->seq); - if (group->rtpoll_states & (1 << PSI_IRQ_FULL)) psi_schedule_rtpoll_work(group, 1, false); - } while ((group = group->parent)); + } + psi_write_end(cpu); } #endif
@@ -1221,12 +1230,14 @@ void psi_cgroup_restart(struct psi_group *group) return;
for_each_possible_cpu(cpu) { - struct rq *rq = cpu_rq(cpu); - struct rq_flags rf; + u64 now;
- rq_lock_irq(rq, &rf); - psi_group_change(group, cpu, 0, 0, true); - rq_unlock_irq(rq, &rf); + guard(rq_lock_irq)(cpu_rq(cpu)); + + psi_write_begin(cpu); + now = cpu_clock(cpu); + psi_group_change(group, cpu, 0, 0, now, true); + psi_write_end(cpu); } } #endif /* CONFIG_CGROUPS */