On Wed, Sep 13, 2023 at 04:24:24PM +0800, Hao Jia wrote:
Igor Raits and Bagas Sanjaya report a RQCF_ACT_SKIP leak warning. Link: https://lore.kernel.org/all/a5dd536d-041a-2ce9-f4b7-64d8d85c86dc@gmail.com
Commit ebb83d84e49b54 ("sched/core: Avoid multiple calling update_rq_clock() in __cfsb_csd_unthrottle()") add RQCF_ACT_SKIP leak warning in rq_clock_start_loop_update(). But this warning is inaccurate and may be triggered incorrectly in the following situations:
CPU0 CPU1__schedule() *rq->clock_update_flags <<= 1;* unregister_fair_sched_group() pick_next_task_fair+0x4a/0x410 destroy_cfs_bandwidth() newidle_balance+0x115/0x3e0 for_each_possible_cpu(i) *i=0* rq_unpin_lock(this_rq, rf) __cfsb_csd_unthrottle()
if (rq->clock_update_flags > RQCF_ACT_SKIP) rf->clock_update_flags = RQCF_UPDATED;
so that preserves all flags, but only stores UPDATED.
raw_spin_rq_unlock(this_rq) rq_lock(*CPU0_rq*, &rf)
rq_pin_lock() rq->clock_update_flags &= (REQ_SKIP|ACT_SKIP); rf->clock_update_flags = 0;
IOW, we preserve ACT_SKIP from CPU0
rq_clock_start_loop_update() rq->clock_update_flags & RQCF_ACT_SKIP <--
And go SPLAT
raw_spin_rq_lock(this_rq)
rq_repin_lock() rq->clock_update_flags |= rf->clock_update_flags;
which restores UPDATED, even though in reality time could have moved on quite significantly.
Anyway....
the purpose of ACT_SKIP is to skip the update (clue in name etc), but the update is very early in __schedule(), but we clear *_SKIP very late, causing it to span that gap above.
Going by the commits that put it there, the thinking was to clear clock_skip_update before unlock, but AFAICT we can clear SKIP flags right after the update_rq_clock() we're wanting to skip, no?
That is, would not something like the below make more sense?
---
diff --git a/kernel/sched/core.c b/kernel/sched/core.c index d8fd29d66b24..bfd2ab4b95da 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -5357,8 +5357,6 @@ context_switch(struct rq *rq, struct task_struct *prev, /* switch_mm_cid() requires the memory barriers above. */ switch_mm_cid(rq, prev, next);
- rq->clock_update_flags &= ~(RQCF_ACT_SKIP|RQCF_REQ_SKIP); - prepare_lock_switch(rq, next, rf);
/* Here we just switch the register state and the stack. */ @@ -6596,6 +6594,8 @@ static void __sched notrace __schedule(unsigned int sched_mode) /* Promote REQ to ACT */ rq->clock_update_flags <<= 1; update_rq_clock(rq); + rq->clock_update_flags &= ~(RQCF_ACT_SKIP|RQCF_REQ_SKIP); +
switch_count = &prev->nivcsw;
@@ -6675,8 +6675,6 @@ static void __sched notrace __schedule(unsigned int sched_mode) /* Also unlocks the rq: */ rq = context_switch(rq, prev, next, &rf); } else { - rq->clock_update_flags &= ~(RQCF_ACT_SKIP|RQCF_REQ_SKIP); - rq_unpin_lock(rq, &rf); __balance_callbacks(rq); raw_spin_rq_unlock_irq(rq);