From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) (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 3055B2FF170 for ; Fri, 20 Feb 2026 16:57:42 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=170.10.129.124 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1771606663; cv=none; b=SgDOSFzQdIwpcizyDXaeWk/EnS9R3ykmZnXFK+nrMvD64vy/iWjG1r3EES3RKgNbog8e51diejLSQbIUSf65/hSyocwnBNT1OGvYgh13bVJY2Gw6WqUwbcwISDainazHvrTZAs5u2xegFZA83YNQqkuSuElp4dYTIie9mxujKys= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1771606663; c=relaxed/simple; bh=mDcQA1kZmkvT9fLI9bJbP+72dmGJKBsoPf+IAsIz7qw=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=TqnMKw32XeIYm7S2XpPVYhtjAN6JZnhETk8NODSN9MnzJ6IYPm5j20N/6Rr+YamoxBpGOR/p7xE14V3XV6uGu+U4DFUwOmBfCOGoVrGkJ3vcu/8fACe5VWds+/IqoWAJM7vZo6BQxsubZb/Ib2GUMW23IW1rCBupuieaF3FFlCI= 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=bhZlI3iy; arc=none smtp.client-ip=170.10.129.124 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="bhZlI3iy" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1771606661; 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=VVlPMF6mEVM5q0VRz5UF5IyvMK8+8R+3jexPuVtyib0=; b=bhZlI3iyEl21IF3QbgSxo1LiLt0ZflysnYT//ptaO+Im81rKXCxvSmyMhbJaHdYwpRzCV2 jlGBXUVXFOEqQpqspGTyxa5+dlEYklNVf6CdrD+ldwiIvvNXK/L+2uOsrJ0EwXdN5ohvAT 98VnR9dL3bF8rnh+wn2U78rqxCmto14= 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-433-oubCWipYPzGnS0az61n1-A-1; Fri, 20 Feb 2026 11:57:38 -0500 X-MC-Unique: oubCWipYPzGnS0az61n1-A-1 X-Mimecast-MFC-AGG-ID: oubCWipYPzGnS0az61n1-A_1771606655 Received: from mx-prod-int-08.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-08.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.111]) (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 38B751956051; Fri, 20 Feb 2026 16:57:35 +0000 (UTC) Received: from tpad.localdomain (unknown [10.96.133.4]) by mx-prod-int-08.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id B39E31800590; Fri, 20 Feb 2026 16:57:33 +0000 (UTC) Received: by tpad.localdomain (Postfix, from userid 1000) id B752A402754C9; Fri, 20 Feb 2026 13:55:57 -0300 (-03) Date: Fri, 20 Feb 2026 13:55:57 -0300 From: Marcelo Tosatti To: Michal Hocko Cc: Leonardo Bras , linux-kernel@vger.kernel.org, cgroups@vger.kernel.org, linux-mm@kvack.org, Johannes Weiner , Roman Gushchin , Shakeel Butt , Muchun Song , Andrew Morton , Christoph Lameter , Pekka Enberg , David Rientjes , Joonsoo Kim , Vlastimil Babka , Hyeonggon Yoo <42.hyeyoo@gmail.com>, Leonardo Bras , Thomas Gleixner , Waiman Long , Boqun Feng , Frederic Weisbecker Subject: Re: [PATCH 0/4] Introduce QPW for per-cpu operations Message-ID: References: <20260206143430.021026873@redhat.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: X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.111 On Fri, Feb 20, 2026 at 01:51:13PM -0300, Marcelo Tosatti wrote: > On Mon, Feb 16, 2026 at 12:00:55PM +0100, Michal Hocko wrote: > > On Sat 14-02-26 19:02:19, Leonardo Bras wrote: > > > On Wed, Feb 11, 2026 at 05:38:47PM +0100, Michal Hocko wrote: > > > > On Wed 11-02-26 09:01:12, Marcelo Tosatti wrote: > > > > > On Tue, Feb 10, 2026 at 03:01:10PM +0100, Michal Hocko wrote: > > > > [...] > > > > > > What about !PREEMPT_RT? We have people running isolated workloads and > > > > > > these sorts of pcp disruptions are really unwelcome as well. They do not > > > > > > have requirements as strong as RT workloads but the underlying > > > > > > fundamental problem is the same. Frederic (now CCed) is working on > > > > > > moving those pcp book keeping activities to be executed to the return to > > > > > > the userspace which should be taking care of both RT and non-RT > > > > > > configurations AFAICS. > > > > > > > > > > Michal, > > > > > > > > > > For !PREEMPT_RT, _if_ you select CONFIG_QPW=y, then there is a kernel > > > > > boot option qpw=y/n, which controls whether the behaviour will be > > > > > similar (the spinlock is taken on local_lock, similar to PREEMPT_RT). > > > > > > > > My bad. I've misread the config space of this. > > > > > > > > > If CONFIG_QPW=n, or kernel boot option qpw=n, then only local_lock > > > > > (and remote work via work_queue) is used. > > > > > > > > > > What "pcp book keeping activities" you refer to ? I don't see how > > > > > moving certain activities that happen under SLUB or LRU spinlocks > > > > > to happen before return to userspace changes things related > > > > > to avoidance of CPU interruption ? > > > > > > > > Essentially delayed operations like pcp state flushing happens on return > > > > to the userspace on isolated CPUs. No locking changes are required as > > > > the work is still per-cpu. > > > > > > > > In other words the approach Frederic is working on is to not change the > > > > locking of pcp delayed work but instead move that work into well defined > > > > place - i.e. return to the userspace. > > > > > > > > Btw. have you measure the impact of preempt_disbale -> spinlock on hot > > > > paths like SLUB sheeves? > > > > > > Hi Michal, > > > > > > I have done some study on this (which I presented on Plumbers 2023): > > > https://lpc.events/event/17/contributions/1484/ > > > > > > Since they are per-cpu spinlocks, and the remote operations are not that > > > frequent, as per design of the current approach, we are not supposed to see > > > contention (I was not able to detect contention even after stress testing > > > for weeks), nor relevant cacheline bouncing. > > > > > > That being said, for RT local_locks already get per-cpu spinlocks, so there > > > is only difference for !RT, which as you mention, does preemtp_disable(): > > > > > > The performance impact noticed was mostly about jumping around in > > > executable code, as inlining spinlocks (test #2 on presentation) took care > > > of most of the added extra cycles, adding about 4-14 extra cycles per > > > lock/unlock cycle. (tested on memcg with kmalloc test) > > > > > > Yeah, as expected there is some extra cycles, as we are doing extra atomic > > > operations (even if in a local cacheline) in !RT case, but this could be > > > enabled only if the user thinks this is an ok cost for reducing > > > interruptions. > > > > > > What do you think? > > > > The fact that the behavior is opt-in for !RT is certainly a plus. I also > > do not expect the overhead to be really be really big. To me, a much > > more important question is which of the two approaches is easier to > > maintain long term. The pcp work needs to be done one way or the other. > > Whether we want to tweak locking or do it at a very well defined time is > > the bigger question. > > Without patchset: > ================ > > [ 1188.050725] kmalloc_bench: Avg cycles per kmalloc: 159 > > With qpw patchset, CONFIG_QPW=n: > ================================ > > [ 50.292190] kmalloc_bench: Avg cycles per kmalloc: 163 > > With qpw patchset, CONFIG_QPW=y, qpw=0: > ======================================= > > [ 29.872153] kmalloc_bench: Avg cycles per kmalloc: 170 > > > With qpw patchset, CONFIG_QPW=y, qpw=1: > ======================================== > > [ 37.494687] kmalloc_bench: Avg cycles per kmalloc: 190 > > With PREEMPT_RT enabled, qpw=0: > =============================== > > [ 65.163251] kmalloc_bench: Avg cycles per kmalloc: 181 > > With PREEMPT_RT enabled, no patchset: > ===================================== > [ 52.701639] kmalloc_bench: Avg cycles per kmalloc: 185 > > With PREEMPT_RT enabled, qpw=1: > ============================== > > [ 35.103830] kmalloc_bench: Avg cycles per kmalloc: 196 #include #include #include #include #include #include #include MODULE_LICENSE("GPL"); MODULE_AUTHOR("Gemini AI"); MODULE_DESCRIPTION("A simple kmalloc performance benchmark"); static int size = 64; // Default allocation size in bytes module_param(size, int, 0644); static int iterations = 1000000; // Default number of iterations module_param(iterations, int, 0644); static int __init kmalloc_bench_init(void) { void **ptrs; cycles_t start, end; uint64_t total_cycles; int i; pr_info("kmalloc_bench: Starting test (size=%d, iterations=%d)\n", size, iterations); // Allocate an array to store pointers to avoid immediate kfree-reuse optimization ptrs = vmalloc(sizeof(void *) * iterations); if (!ptrs) { pr_err("kmalloc_bench: Failed to allocate pointer array\n"); return -ENOMEM; } preempt_disable(); start = get_cycles(); for (i = 0; i < iterations; i++) { ptrs[i] = kmalloc(size, GFP_ATOMIC); } end = get_cycles(); total_cycles = end - start; preempt_enable(); pr_info("kmalloc_bench: Total cycles for %d allocs: %llu\n", iterations, total_cycles); pr_info("kmalloc_bench: Avg cycles per kmalloc: %llu\n", total_cycles / iterations); // Cleanup for (i = 0; i < iterations; i++) { kfree(ptrs[i]); } vfree(ptrs); return 0; } static void __exit kmalloc_bench_exit(void) { pr_info("kmalloc_bench: Module unloaded\n"); }