From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.133.124]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id B99274F4024 for ; Wed, 14 Oct 2026 10:59:16 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=170.10.133.124 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1791975558; cv=none; b=HAa0EfLhrmT9k3XBfJof8ZztTtpcmlkt03kmUmkCjh4g8324xS6yEc/FfWk4yHkn7EfU43yWOVb5fvSDhvQvaCPufZfeM1dtDLHE037ciPFcdPvAURTJxFjBzhj/ycFnifPNsW64vmSgftmKZiu2G7atlRAr3A/87xxzdXHKV5Q= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1791975558; c=relaxed/simple; bh=QhMbap5+ZpHUhjScg9Gm1HgrZbg/UFZ+K5RzqbsV6Tg=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=dWwpyI80ysXHLP6NZZuJZm5qHrVAkfm1aTd/HMfBR7I6UsPYiiwZShfW+Ydpub0E0tnOQgSkowgiAvtXOFD5neXLuiCq+RqIVmeJfVyzbuxYkNgbzSr4agVT/Fyvr9qkqtZKpsPISMG9PnDGHSPkqAqPQFIXW3d8EC4nkInRDPw= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=redhat.com; spf=pass smtp.mailfrom=redhat.com; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b=EMVFfuJz; arc=none smtp.client-ip=170.10.133.124 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=redhat.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=redhat.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="EMVFfuJz" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1791975555; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=mOLlp/ZrpnArBAnQIiyLuMNq8Q1yVdA+bhnGWdgiJ4E=; b=EMVFfuJzgMXa9UkcabEdBxqOyCoK5DoVMyE5GkqflKugj9S6MPYhDHe2RtBMOzCJ7cG/XU cWVD7rP7YucYKD6cgKs0lOXclux2K4EMh7Q5FW95+LY2bdlMv9FVKS9kUQjJnHD50KAhVR p0kROQicT38RgOiBvJjtasNA3L9K8sM= Received: from mx-prod-mc-01.mail-002.prod.us-west-2.aws.redhat.com (ec2-54-186-198-63.us-west-2.compute.amazonaws.com [54.186.198.63]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-588-CFhV3TTSPne-WzFvwHJuJA-1; Wed, 14 Oct 2026 10:59:12 +0000 X-MC-Unique: CFhV3TTSPne-WzFvwHJuJA-1 X-Mimecast-MFC-AGG-ID: CFhV3TTSPne-WzFvwHJuJA_1791975550 Received: from mx-prod-int-06.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-06.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.93]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mx-prod-mc-01.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 9D0A61956065; Wed, 14 Oct 2026 10:59:09 +0000 (UTC) Received: from pauld.westford.csb (headnet03.pony-001.prod.iad2.dc.redhat.com [10.2.32.114]) by mx-prod-int-06.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 2047A1800480; Wed, 14 Oct 2026 10:59:07 +0000 (UTC) Date: Wed, 14 Oct 2026 06:59:04 -0400 From: Phil Auld To: K Prateek Nayak Cc: "Paul E. McKenney" , Ingo Molnar , Peter Zijlstra , Juri Lelli , Vincent Guittot , Dietmar Eggemann , Steven Rostedt , Ben Segall , Mel Gorman , Valentin Schneider , Joel Fernandes , linux-kernel@vger.kernel.org, kernel-team@meta.com Subject: Re: [PATCH] sched/core: Call wakeup_preempt() for queued tasks during class change Message-ID: <20261014105904.GA699535@pauld.westford.csb> References: <8fa7f795-518c-4507-b684-60b0c1cac2c5@paulmck-laptop> <20261013152455.9039-1-kprateek.nayak@amd.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20261013152455.9039-1-kprateek.nayak@amd.com> X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.93 Hi Prateek, On Tue, Oct 13, 2026 at 03:24:55PM +0000 K Prateek Nayak wrote: > Paul reported recent RCU changes exposed a lost resched where CPU idles > with a rcutorture thread waiting while being completely runnable. > > Dumping the rq state at the time of idling with runnable tasks showed > the following: > > rq->next_class = &fair_sched_class > rq->nr_running = 1 > rq->cfs.h_nr_queued = 0 > rq->rt.rt_nr_running = 1 > > Although the waiting task was an RT task, "rq->next_class" did not point > to rt_sched_class and was stuck at the lower priority fair_sched_class. > > Inspecting sched_change_end() revealed the following race with > __sched_setscheduler() for queued, but non-running tasks: > > CPU0 CPU1 > ==== ==== > > __schedule() > > /* > * rq->donor is a fair task that blocks. > * rq->next_class = &fair_sched_class > */ > > rq_lock(rq0) > > pick_next_task() > /* nr_running = 0 */ > pick_task_fair() > sched_balance_newidle() > raw_spin_rq_unlock(rq0) > rq_lock(rq0) > > activate_task(rq0, p) > /* p is a fair task */ > rq0->nr_running++; > rq0->cfs.h_nr_queued++; > wakeup_preempt(rq, p) > rq->next_class = &fair_sched_class > resched_curr() > > rq_unlock(rq0) > > __sched_setscheduler(p) > /* p is moved to SCHED_RR */ > task_rq_lock(p) > sched_change_begin() > > rq0->cfs.h_nr_queued--; > rq0->rt.rt_nr_running++; > > sched_change_end() > /* > * p->on_rq = 1 (ctx->queued) > * but p is not rq->donor > * (!ctx->running) hence > * resched_curr() is and > * rq->next_class upgrade is > * skipped. > */ > > task_rq_unlock() > > raw_spin_rq_lock(rq0) > > /* > * rq->nr_running = 1 > * rq->cfs.h_nr_queued = 0 > * rq->rt.rt_nr_running = 1 > * rq->next_class = &fair_sched_class > */ > > if (rq->cfs.h_nr_queued && ...) > /* Above is false */ > > if (rq_modified_above(this_rq, &fair_sched_class)) > /* Also false since rq->next_class is still fair */ > > /* > * sched_balance_newidle() is considered to > * have failed to pull any fair tasks and no > * other wakeups were detected. > */ > return NULL; /* pick_task_fair() */ > > pick_task_idle() > return rq->idle; > > next = rq->idle; > > /* Preemption indicators are lost */ > clear_tsk_need_resched(prev); > clear_preempt_need_resched(); > > rq->curr = rq->idle; > > !!! rq->idle switches in with rq->running == 1 but !need_resched() !!! > > In simple words, enqueue, followed by a class change of the newly queued > task goes unnoticed in rq->next_class indicator and > sched_balance_newidle() sees no fair task, nor any higher priority task > enqueued. > > The above sequence will lead to CPU idling with a runnable task on the > CPU. > > Call wakeup_preempt() for ctx->queued cases in sched_change_end() to > re-evaluate rq->next_class after class change (ENQUEUE_CLASS) and set > the NEED_RESCHED indicator accordingly. > > Newidle balance will correctly see an enqueue of a higher priority > task if a queued task was upgraded when the rq_lock() was dropped and > will retry the pick. > > Reported-by: "Paul E. McKenney" > Closes: https://lore.kernel.org/lkml/8fa7f795-518c-4507-b684-60b0c1cac2c5@paulmck-laptop/ > Fixes: 1e900f415c60 ("sched: Detect per-class runqueue changes") > Signed-off-by: K Prateek Nayak Nice. That rq lock dropping in schedule() has been the cause of quite a handful of "interesting" outcomes hasn't it? :) I think this makes the code cleaner too. Reviewed-by: Phil Auld Cheers, Phil > --- > Patches are based on tip:sched/urgent at commit a0bb6fac53fa > ("sched/core: Account PSI IRQ time to the execution context, not the > scheduling context"). > > Tested with sched-messaging and Paul's TREE01 reproducer. > --- > kernel/sched/core.c | 19 +++++++++---------- > 1 file changed, 9 insertions(+), 10 deletions(-) > > diff --git a/kernel/sched/core.c b/kernel/sched/core.c > index 1fe40de6ebe3..1754d9f95f2c 100644 > --- a/kernel/sched/core.c > +++ b/kernel/sched/core.c > @@ -11327,17 +11327,16 @@ void sched_change_end(struct sched_change_ctx *ctx) > if (p->sched_class->switched_to) > p->sched_class->switched_to(rq, p); > > + /* > + * Check if the task can preempt the current in case the class > + * was upgraded. Note that none of the switch*_from() methods > + * know the new class and none of the switch*_to() methods know > + * the old class so we must resched here. > + */ > + if (ctx->queued) > + wakeup_preempt(rq, p, 0); > + > if (ctx->running) { > - /* > - * If this was a class promotion; let the old class > - * know it got preempted. Note that none of the > - * switch*_from() methods know the new class and none > - * of the switch*_to() methods know the old class. > - */ > - if (sched_class_above(p->sched_class, ctx->class)) { > - rq->next_class->wakeup_preempt(rq, p, 0); > - rq->next_class = p->sched_class; > - } > /* > * If this was a degradation in class; make sure to > * reschedule. > > base-commit: a0bb6fac53fa7cf1cadb487b43d4c9276a6b82e3 > -- > 2.34.1 > > --