Re: [PATCH 2/6] sched: Simplify migration_cpu_stop()
From: Valentin Schneider
Date: Wed Feb 24 2021 - 10:50:13 EST
On 24/02/21 13:24, Peter Zijlstra wrote:
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@@ -1950,31 +1931,20 @@ static int migration_cpu_stop(void *data
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goto out;
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if (pending) {
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- p->migration_pending = NULL;
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+ if (p->migration_pending == pending)
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+ p->migration_pending = NULL;
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complete = true;
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}
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- /* migrate_enable() -- we must not race against SCA */
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- if (dest_cpu < 0) {
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- /*
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- * When this was migrate_enable() but we no longer
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- * have a @pending, a concurrent SCA 'fixed' things
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- * and we should be valid again. Nothing to do.
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- */
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- if (!pending) {
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- WARN_ON_ONCE(!cpumask_test_cpu(task_cpu(p), &p->cpus_mask));
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- goto out;
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- }
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-
This is fixed by 5+6, but at this patch I think you can have double
completions - I thought this was an issue, but briefly looking at
completion stuff it might not. In any case, consider:
task_cpu(p) == Y
SCA(p, X);
SCA(p, Y);
SCA(p, Y) will uninstall SCA(p, X)'s pending and complete.
migration/Y kicked by SCA(p, X) will grab arg->pending, which is still
SCA(p, X)'s pending and also complete.
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+ if (dest_cpu < 0)
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dest_cpu = cpumask_any_distribute(&p->cpus_mask);
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- }
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if (task_on_rq_queued(p))
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rq = __migrate_task(rq, &rf, p, dest_cpu);
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else
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p->wake_cpu = dest_cpu;
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- } else if (dest_cpu < 0 || pending) {
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+ } else if (pending) {
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/*
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* This happens when we get migrated between migrate_enable()'s
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* preempt_enable() and scheduling the stopper task. At that