From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (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 56AF7283CAF for ; Thu, 10 Apr 2025 14:20:14 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1744294815; cv=none; b=WatdxuHNCe9O83AWc2WY1a6xaVG0rIcAIQ98fl90GujScmqEo12f+3Y81FRLCIxuq11TYgTD8+feq0FWVuN9laZgqdiExDrodJ34y9xFsgTw5Q2SzwcLWK1cvn9iLEsmbxtKTO3+mSmI5dx5hGhA005q1GlTPLh37SD1MJQs0P8= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1744294815; c=relaxed/simple; bh=JattNGha423Yy6m0LWWQusIZ0pJwwQ/7X2UwSM6+8fY=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=A0SsIABSv6IOr/LFvh9ZU0ZIiXMUK0GcakKoIsm8xkEEtPHZC6i7nMvFUxLgA16wt2eMG84WSW1R5F0hKdfLZfkH/4fQ6bz61O3HGrhJOZTEoflDxdAXEvZA2ewt9UfoloCeQ8/TX3szU3KeGBaosHDOCTEf++hNa+JP/LPspKM= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=dFF0SloR; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="dFF0SloR" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 66B05C4CEDD; Thu, 10 Apr 2025 14:20:14 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1744294814; bh=JattNGha423Yy6m0LWWQusIZ0pJwwQ/7X2UwSM6+8fY=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=dFF0SloRdMIr4BsE6DxXf2OZ3CABG+q7v8vewQCmqpU+gWTnZdsdzKgMVtM/3E2eO ykZ5JUiTRGpItd3LOyt3JNSymwJ0mJXuoJo8Bd4vyZ4ZnnqDuAq0ULZH+fWuSZqTvx ItDsPYw/RWtTHJg9MCjFXRYar4nTYcE2H1vpSVV8+26V8D/BhM8TJr9e/2pwnxIwSn PjeZ5A8k1DhbOBK8/s62E6fCizsUBl3pYrAYhzWFOxuuj5juP6dmxeXKWB9Et7Qc0q FypgVdkQcIwGdbKBicbHZphPHUBJH2ZfUz8NYmvAV8ZevzzR1G2TxqknZOUjGUL05Q JC/LOTd9/tPhw== Date: Thu, 10 Apr 2025 16:20:11 +0200 From: Frederic Weisbecker To: Gabriele Monaco Cc: Thomas Gleixner , linux-kernel@vger.kernel.org, Waiman Long Subject: Re: [PATCH] timers: Exclude isolated cpus from timer migation Message-ID: References: <20250410065446.57304-2-gmonaco@redhat.com> <87ecy0tob1.ffs@tglx> <2c9d71fd79d7d1cec66e48bcb87b39a874858f01.camel@redhat.com> <87wmbsrwca.ffs@tglx> <4fdc6582c828fbcd8c6ad202ed7ab560134d1fc3.camel@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=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <4fdc6582c828fbcd8c6ad202ed7ab560134d1fc3.camel@redhat.com> Le Thu, Apr 10, 2025 at 03:56:02PM +0200, Gabriele Monaco a écrit : > On Thu, 2025-04-10 at 15:27 +0200, Frederic Weisbecker wrote: > > But how do we handle global timers that have been initialized and > > queued from > > isolated CPUs? > > I need to sketch a bit more the solution but the rough idea is: > 1. isolated CPUs don't pull remote timers That's the "easy" part. > 2. isolated CPUs ignore their global timers and let others pull them > perhaps with some more logic to avoid it expiring This will always involve added overhead because you may need to wake up a CPU upon enqueueing a global timer to make sure it will be handled. At least when all other CPUs are idle. > Wouldn't that be sufficient? > > Also, I would definitely do 1. for any kind of isolation, but I'm not > sure about 2. > Strictly speaking domain isolated cores don't claim to be free of > kernel noise, even if they initiate it (but nohz_full ones do). > What would be the expectation there? I don't know, I haven't heard complains about isolcpus handling global timers so far... I wouldn't pay much attention to 2) until anybody complains. Does 1) even matter to anybody outside nohz_full ? Thanks. -- Frederic Weisbecker SUSE Labs