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 301942853FA for ; Thu, 10 Apr 2025 13:27:11 +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=1744291632; cv=none; b=WxZF4CymXaDNByydWlJ2/mN1IZ9Q+RTmTHcJvgvjV/6Pf5IEDlI9hYAQJ2t5h/hWBiF7fMXL/VQlSOYQmAmgxRX/HqgOLK9w1DtM4Y5FTEi8h3Y1NHhiC4YZUB+ZKghiOdEIR+P9cuM9Ls6bxPNDTKeVp4lS8ehV2YiA3q+Svjw= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1744291632; c=relaxed/simple; bh=3gPfsEXtcRb/g3NXNMRegFnbtp1ZB4qYMxP7JSx39YM=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=mGBL98goCxYfVBa6NafSMHJco2EfHcQAC2goBwPtHKZBkvqlxtbANwjyrNCoJqPnBhErYQSkX0ke9QX04dlboyFYcEg6M0YIy1D4+obzwkI3gd0+1F1rL3eR7naJs6pDQK9HSAyopFTl4UaetmZrk1vh9AcvgGXhagQY22REGRI= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=bJYwGj1D; 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="bJYwGj1D" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 43820C4CEDD; Thu, 10 Apr 2025 13:27:11 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1744291631; bh=3gPfsEXtcRb/g3NXNMRegFnbtp1ZB4qYMxP7JSx39YM=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=bJYwGj1Dm3McX0ECCISJLFD1dSHF40isk5OWzxQOYfBlODk553AVJAna7I3h0YDoW L/pFMhyFSHm4FfprhT91HSttTHz//EU2HHHlFOF/mwluuAhCj8mWkfZxzsIqWUwZgB OskGCi7r2TI0e0awqdV8xLupCnq+qYgHUs/uTzPKfaejgKDUny8U34dRGFIxqoqK4p iuZlZApfKY3H+4723azGkpF/Q2XA5w9+F5xFcIYKQcJQ+AudGrqGgtVm+HfTnic525 yaSX8QhcFtqulEkMkFR+4L6n+0QZetyknVLQ096MgVqtX/Vx0vtyvv1sHo1hwpDvQM hCpiSSspEyZ7A== Date: Thu, 10 Apr 2025 15:27:08 +0200 From: Frederic Weisbecker To: Thomas Gleixner Cc: Gabriele Monaco , 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> 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: <87wmbsrwca.ffs@tglx> Le Thu, Apr 10, 2025 at 03:15:49PM +0200, Thomas Gleixner a écrit : > On Thu, Apr 10 2025 at 15:03, Frederic Weisbecker wrote: > > Le Thu, Apr 10, 2025 at 12:38:25PM +0200, Gabriele Monaco a écrit : > > Speaking of, those are two different issues here: > > > > * nohz_full CPUs are handled just like idle CPUs. Once the tick is stopped, > > the global timers are handled by other CPUs (housekeeping). There is always > > one housekeeping CPU that never goes idle. > > One subtle thing though: if the nohz_full CPU fires a tick, because there > > is a local timer to be handled for example, it will also possibly handle > > some global timers along the way. If it happens to be a problem, it should > > be easy to resolve. > > > > * Domain isolated CPUs are treated just like other CPUs. But there is not > > always a housekeeping CPU around. And no guarantee that there is always > > a non-idle CPU to take care of global timers. > > That's an insianity. It works, but it doesn't make much sense arguably. > > >> Thinking about it now, since global timers /can/ start on isolated > >> cores, that makes them quite different from offline ones and probably > >> considering them the same is just not the right thing to do.. > >> > >> I'm going to have a deeper thought about this whole approach, perhaps > >> something simpler just preventing migration in that one direction would > >> suffice. > > > > I think we can use your solution, which involves isolating the CPU from tmigr > > hierarchy. And also always queue global timers to non-isolated targets. > > Why do we have to inflict extra complexity into the timer enqueue path > instead of preventing the migration to, but not the migration from > isolated CPUs? But how do we handle global timers that have been initialized and queued from isolated CPUs? Thanks. -- Frederic Weisbecker SUSE Labs