From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from e28smtp03.in.ibm.com (e28smtp03.in.ibm.com [122.248.162.3]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (No client certificate requested) by lists.ozlabs.org (Postfix) with ESMTPS id 0938D1A0008 for ; Wed, 18 Mar 2015 02:49:42 +1100 (AEDT) Received: from /spool/local by e28smtp03.in.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Tue, 17 Mar 2015 21:19:40 +0530 Received: from d28relay01.in.ibm.com (d28relay01.in.ibm.com [9.184.220.58]) by d28dlp02.in.ibm.com (Postfix) with ESMTP id 80E0C394004E for ; Tue, 17 Mar 2015 21:19:37 +0530 (IST) Received: from d28av01.in.ibm.com (d28av01.in.ibm.com [9.184.220.63]) by d28relay01.in.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id t2HFna8934340958 for ; Tue, 17 Mar 2015 21:19:37 +0530 Received: from d28av01.in.ibm.com (localhost [127.0.0.1]) by d28av01.in.ibm.com (8.14.4/8.14.4/NCO v10.0 AVout) with ESMTP id t2HFnau4025605 for ; Tue, 17 Mar 2015 21:19:36 +0530 Message-ID: <55084D0C.1040309@linux.vnet.ibm.com> Date: Tue, 17 Mar 2015 21:19:32 +0530 From: Shreyas B Prabhu MIME-Version: 1.0 To: Benjamin Herrenschmidt , Michael Ellerman Subject: Re: [RFC, v2] powerpc/powernv: Introduce kernel param to control fastsleep workaround behavior References: <20150317085708.7102714017B@ozlabs.org> <1426585198.4770.23.camel@kernel.crashing.org> In-Reply-To: <1426585198.4770.23.camel@kernel.crashing.org> Content-Type: text/plain; charset=utf-8 Cc: linuxppc-dev@lists.ozlabs.org, Paul Mackerras , linux-kernel@vger.kernel.org List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , On Tuesday 17 March 2015 03:09 PM, Benjamin Herrenschmidt wrote: > On Tue, 2015-03-17 at 19:57 +1100, Michael Ellerman wrote: >> >> So my first preference is that you just bite the bullet and decide to either >> always apply the workaround, or just stick with the current behaviour. That's a >> trade-off between (I think) better idle latency but a risk of checkstops, vs. >> slower idle latency but less (how much less?) risk of checkstops. >> >> I think the reason you're proposing a kernel parameter is because we aren't >> willing to make that decision, ie. we're saying that users should decide. Is >> that right? > > Correct. More specifically, a fairly high profile user that I will not > name here has expressed interest in such a feature... > >> I'm not a big fan of kernel parameters. They are a pain to use, and are often >> just pushing a decision down one layer for no reason. What I mean is that >> individual users are probably just going to accept whatever the default value >> is from their distro. > > Right. This is quite an obscure tunable. > >> But anyway, that's a bit of a rant. >> >> As far as this patch is concerned, I don't think it actually needs to be a >> kernel parameter. >> >> >From what I can see below, the decision as to whether you apply the workaround >> or not doesn't affect the list of idle states. So this could just as well be a >> runtime parameter, ie. a sysfs file, which can then be set by the user whenever >> they like? They might do it in a boot script, but that's up to them. > > Right, that would work too. Okay. I'll send a patch with this design. > >> For simplicity I think it would also be fine to make it a write-once parameter, >> ie. you don't need to handle undoing it. > > It would be easy enough to make it rw using stop machine I think... > >> I think the only complication that would add is that you'd need to be a little >> careful about the order in which you nop out the calls vs applying the >> workaround, in case some threads are idle when you're called. Right, we should be safe with this sequence- - NOP call to undo workaround - Apply workaround on all cores. - NOP call to apply workaround > > I wouldn't bother with NOP'ing in that case, a runtime test will probably be noise > in the measurement. > Didn't get your point here. Do you mean, ignore the request if some cores are in sleep or deeper state? > Cheers, > Ben. > >