Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1761218Ab3JPWhz (ORCPT ); Wed, 16 Oct 2013 18:37:55 -0400 Received: from b232-131.smtp-out.amazonses.com ([199.127.232.131]:37290 "EHLO b232-131.smtp-out.amazonses.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1758776Ab3JPWhy (ORCPT ); Wed, 16 Oct 2013 18:37:54 -0400 Date: Wed, 16 Oct 2013 22:37:52 +0000 From: Christoph Lameter X-X-Sender: cl@gentwo.org To: Andrew Morton cc: Frederic Weisbecker , Mike Galbraith , Thomas Gleixner , Gilad Ben-Yossef , "linux-kernel@vger.kernel.org" , "Paul E. McKenney" , Mike Frysinger Subject: Re: [PATCH] kmod: Run usermodehelpers only on cpus allowed for kthreadd In-Reply-To: <20131016141326.2e517e18e4d8af880c97a282@linux-foundation.org> Message-ID: <00000141c36b03b6-2f9bd3de-d153-4359-901f-084aeb4c040d-000000@email.amazonses.com> References: <00000141c1b99b20-64f9d142-961a-447e-8ebe-40f86b638278-000000@email.amazonses.com> <20131016141326.2e517e18e4d8af880c97a282@linux-foundation.org> User-Agent: Alpine 2.02 (DEB 1266 2009-07-14) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII X-SES-Outgoing: 199.127.232.131 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1370 Lines: 36 On Wed, 16 Oct 2013, Andrew Morton wrote: > > usermodehelper() threads can currently run on all processors. > > This is an issue for low latency cores. > > How much of an issue? The severity of the problem is utterly unclear > from this description. This causes a context switch and processing to occur. Can take up to microseconds which is a pretty long timeframe when decisions have to be made in microseconds by active processing. This will cause a trade to fail. > > - /* We can run anywhere, unlike our parent keventd(). */ > > - set_cpus_allowed_ptr(current, cpu_all_mask); > > + /* We can run anywhere kthreadd can run */ > > This is a poor comment - it explains "what" (which was utterly obvious) > but doesn't explain "why". The reader will want to know *why* > call_usermodehelper() only runs on kthreadd CPUs, but we didn't tell > him. We'd like to have the ability to avoid running usermodehelper on certain cpus to avoid cpu holdoff situations? Would that we an acceptable explanation? Or restricting kthreadd will also restrict usermodehelper spawning to allow control for all spawned kernel threads? -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/