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 447651EDA2C for ; Tue, 16 Sep 2025 11:32:28 +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=1758022351; cv=none; b=MHwFsFhpSO0c4PjpFY8rXtLM+YUhFjurOl+Gca90CbgTbOoRCBW7+9KgOURWnJR9HLPIoIa48bQ/uJ+mINW5TGGs4v3F4R86nX1QHqjVhG1ERwErWcB7nZEPP1LB8JeQHkrze8Kg9fyMz5aTFcpJ4WhieD9jFrgRf1jQKcj3aWk= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1758022351; c=relaxed/simple; bh=qrKm7Hjc2f6pjKnmdlrxFhFRJ74oWb1RCb7bzHCChWo=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=sBLHMG5/42hrx23C0ickPufuE0hAUCg11DJpbKrBDWG0B/dZnCOXp5iOynB5I6QJUtoprtxpD0Wa+xat/lQ9KV3//n//q8qMUghM26bibSOL8L0KSX2vmmKhyfqmHLTP2Cjqpa13uizJQKpT4758/1I6u9hYo9Jv3HH2+OTTz7Y= 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=VvnILw2n; 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="VvnILw2n" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1758022348; 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=9hCLZ50vYlDxaPnHwAZOwSPKZAjcTojGhWt+bNONuqk=; b=VvnILw2npz5dsOBFwt7dOpOVduRHIoHNV3wiIM5SqFR4sdlaWqcIR0oUYVBfibnhSIxwvl UffO0Fp0JCBqbK23m7sA6t1Nz51Go9yn90pDSARhOBQA5i54VijUe0a39cPHhrLPTV23Ro YTbmfQhpuAyOznIlzUAtUGPiMNHaJq8= Received: from mx-prod-mc-06.mail-002.prod.us-west-2.aws.redhat.com (ec2-35-165-154-97.us-west-2.compute.amazonaws.com [35.165.154.97]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-140-AtP0HQ_5NM6pJLds3SjhZg-1; Tue, 16 Sep 2025 07:32:25 -0400 X-MC-Unique: AtP0HQ_5NM6pJLds3SjhZg-1 X-Mimecast-MFC-AGG-ID: AtP0HQ_5NM6pJLds3SjhZg_1758022343 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-06.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 1983F1800298; Tue, 16 Sep 2025 11:32:22 +0000 (UTC) Received: from dhcp-27-174.brq.redhat.com (unknown [10.45.224.65]) by mx-prod-int-06.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with SMTP id 7A2121800447; Tue, 16 Sep 2025 11:32:14 +0000 (UTC) Received: by dhcp-27-174.brq.redhat.com (nbSMTP-1.00) for uid 1000 oleg@redhat.com; Tue, 16 Sep 2025 13:30:58 +0200 (CEST) Date: Tue, 16 Sep 2025 13:30:49 +0200 From: Oleg Nesterov To: Peter Zijlstra Cc: "Luis Claudio R. Goncalves" , Sebastian Andrzej Siewior , Clark Williams , Steven Rostedt , Tejun Heo , David Vernet , Barret Rhoden , Josh Don , Crystal Wood , linux-kernel@vger.kernel.org, linux-rt-devel@lists.linux.dev, Juri Lelli , Ben Segall , Dietmar Eggemann , Ingo Molnar , Mel Gorman , Valentin Schneider , Vincent Guittot , Thomas Gleixner , Wander Lairson Costa Subject: Re: [RESEND PATCH] sched: restore the behavior of put_task_struct() for non-rt Message-ID: <20250916113043.GA32038@redhat.com> References: <20250915113812.GB3245006@noisy.programming.kicks-ass.net> <20250915123539.GC23082@redhat.com> <20250916100953.GG3245006@noisy.programming.kicks-ass.net> Precedence: bulk X-Mailing-List: linux-rt-devel@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20250916100953.GG3245006@noisy.programming.kicks-ass.net> User-Agent: Mutt/1.5.24 (2015-08-30) X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.93 On 09/16, Peter Zijlstra wrote: > > On Mon, Sep 15, 2025 at 02:35:40PM +0200, Oleg Nesterov wrote: > > On 09/15, Peter Zijlstra wrote: > > > > > > Why have !RT behave differently? That is, why isn't this simply a > > > 'buggy' comment/changelog issue? > > > > Well, this was discussed several times, in particular see > > https://lore.kernel.org/all/CAHk-=whtj+aSYftniMRG4xvFE8dmmYyrqcJyPmzStsfj5w9r=w@mail.gmail.com/ > > > > And task_struct->rcu_users was introduced to avoid RCU call in put_task_struct() ... > > Ah, I forgot about that thing.. Although I had vague memories of that > argument on rcu_assign_pointer() vs RCU_INIT_POINTER(). > > > But I won't really argue if you decide to remove this !RT optimization, although > > I think it would be better to do this in a separate patch. > > Right. So when they wanted to remove that preemptible() clause, I was > like why again do we have this !RT exception at all, and can't we get > rid of that. > > If git isn't confusing me again, this got merged in this cycle. But so > far no benchmark came and told us this was a bad idea. I still think it would be safer to merge this patch from Luis before v6.17, then possibly remove this special case in a separate patch... > So what do we do now... do we restore the !RT exception (so far there > aren't any numbers to suggest this mattered) or do we let it be for a > bit and then go and clean things up? It is not that simple. Please note that put_task_struct_rcu_user() delays put(tsk->usage), not free(tsk). So for example with this change > @@ -305,7 +288,7 @@ void release_task(struct task_struct *p) > if (thread_group_leader(p)) > flush_sigqueue(&p->signal->shared_pending); > > - put_task_struct_rcu_user(p); > + put_task_struct(p); > > p = leader; > if (unlikely(zap_leader)) This code rcu_read_lock(); tsk = find_task_by_vpid(...); if (tsk) get_task_struct(tsk); rcu_read_unlock(); becomes wrong, get_task_struct(tsk) can increment tsk->usage == 0. Oleg.