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 459393A1A28 for ; Tue, 24 Feb 2026 18:26:33 +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=1771957597; cv=none; b=EbTRarkiDSNl8Ne4b/TIP6eq2YVyOkEOXxCC4OtS1M1gQO0gpgWpVRH8EYqrbatNpncuKodYUp7NLkO3r8AJ32hSso4/PDIDFzVMoAFFpR566nUD6XK+ipVdE8/mCN/ppGk99ze8YH78CNCBzz9EAOH7QvLMwOJPKX20dxKxwW4= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1771957597; c=relaxed/simple; bh=ujg4JwIYLvspkvuyEEPh1L3eQ2IzutQT2hoqTvgB6zA=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=V+hNH2WYlG26+hNuMMHFJAPVKPlpHoldqkOq6VknGrqUGze6yFLRa4L/klTSQAXlsZnf+XgNKvPE96sJ8WfsKYWBQytnOoKWBtxyeilrn7OYf3SgReUEBonbPAO2lm+b7XlDwlnIiH8pMXuCFRdM+v0Mk9zXh2nmbnkZQnF/9aQ= 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=ikHv1j9R; 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="ikHv1j9R" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1771957592; 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=7Uf5EJf+Tw5nbDLapVKw8M9tfsonMMBNq6SuwMEXglI=; b=ikHv1j9RgVuBVPSnBFHJihg8CIcM7hkeclLRTYYhYaPcc1yp8i+mAhvbplMRyAdWS5VgfB bTl0OqaMUNTnHovMrvkK/UprpvsQX2mNz+fhzKAV6pOj9ibU7j4QgJRon+tJWxAK0IWG7Z 7frj3xoDQjTSEKUf63z+3/3sGRw3/B0= Received: from mx-prod-mc-08.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-414-58eZhZIcMliTHgOPkvQPJA-1; Tue, 24 Feb 2026 13:26:26 -0500 X-MC-Unique: 58eZhZIcMliTHgOPkvQPJA-1 X-Mimecast-MFC-AGG-ID: 58eZhZIcMliTHgOPkvQPJA_1771957582 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-08.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 08FF21800578; Tue, 24 Feb 2026 18:26:22 +0000 (UTC) Received: from tpad.localdomain (unknown [10.96.133.3]) by mx-prod-int-06.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 2CF1C1800370; Tue, 24 Feb 2026 18:26:19 +0000 (UTC) Received: by tpad.localdomain (Postfix, from userid 1000) id CBBF340164D27; Mon, 23 Feb 2026 08:20:51 -0300 (-03) Date: Mon, 23 Feb 2026 08:20:51 -0300 From: Marcelo Tosatti To: Michal Hocko Cc: Vlastimil Babka , 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: <3f2b985a-2fb0-4d63-9dce-8a9cad8ce464@suse.com> Precedence: bulk X-Mailing-List: cgroups@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 10.30.177.93 On Mon, Feb 23, 2026 at 10:11:11AM +0100, Michal Hocko wrote: > On Fri 20-02-26 16:01:59, Marcelo Tosatti wrote: > > On Fri, Feb 20, 2026 at 06:58:10PM +0100, Vlastimil Babka wrote: > [...] > > > >> So if we can assume that workloads on isolated cpus make syscalls only > > > >> rarely, and when they do they can tolerate them being slower, I think the > > > >> "avoid sheaves on isolated cpus" would be the best way here. > > > > > > > > I am not sure its safe to assume that. Ask Gemini about isolcpus use > > > > cases and: > > > > > > I don't think it's answering the question about syscalls. But didn't read > > > too closely given the nature of it. > > > > People use isolcpus with all kinds of programs. > > > > > > For example, AF_XDP bypass uses system calls (and wants isolcpus): > > > > > > > > https://www.quantvps.com/blog/kernel-bypass-in-hft?srsltid=AfmBOoryeSxuuZjzTJIC9O-Ag8x4gSwjs-V4Xukm2wQpGmwDJ6t4szuE > > > > > > Didn't spot system calls mentioned TBH. > > > > I don't see why you want to reduce performance of applications that > > execute on isolcpus=, if you can avoid that. > > If you can avoid that by making performance bad for everybody else then > then it seems safer to sacrifice those workloads that are much more > special - i.e. cpu isolation. > -- > Michal Hocko > SUSE Labs Performance is not bad for everyone else: 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 And its probably possible to remove those 4 cycles. Which makes reduction of performance of isolcpus not necessary.