From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Message-ID: <42578019.7050501@yahoo.com.au> Date: Sat, 09 Apr 2005 17:11:21 +1000 From: Nick Piggin MIME-Version: 1.0 Subject: Re: [patch] sched: unlocked context-switches References: <20050409043848.GA2677@elte.hu> <42577602.8090507@yahoo.com.au> <20050409065507.GA4866@elte.hu> In-Reply-To: <20050409065507.GA4866@elte.hu> Content-Type: text/plain; charset=us-ascii; format=flowed Content-Transfer-Encoding: 7bit To: Ingo Molnar Cc: "Luck, Tony" , linux-kernel@vger.kernel.org, Andrew Morton , linux-arch@vger.kernel.org List-ID: Ingo Molnar wrote: > * Nick Piggin wrote: > > >>I did propose doing unconditionally unlocked switches a while back >>when my patch first popped up - you were against it then, but I guess >>you've had second thoughts? > > > the reordering of switch_to() and the switch_mm()-related logic was that > made it really worthwile and clean. I.e. we pick a task atomically, we > switch stacks, and then we switch the MM. Note that this setup still > leaves the possibility open to move the stack-switching back under the > irq-disabled section in a natural way. > Yeah true. I didn't come up with code for you to look at at that point anyway so you were obviously just speculating! > >>It does add an extra couple of stores to on_cpu, and a wmb() for >>architectures that didn't previously need the unlocked switches. And >>ia64 needs the extra interrupt disable / enable. Probably worth it? > > > it also removes extra stores to rq->prev_mm and other stores. I havent > measured any degradation on x86. > Yeah true, although that is just a single cacheline (which will be hot for any context switch heavy workload). On the other hand, I tried put oncpu near other fields that are accessed during context switch, so maybe its not an issue. > If the irq disable/enable becomes widespread i'll do another patch to > push the irq-enabling into switch_to() so the arch can do the > stack-switch first and then enable interrupts and do the rest - but i > didnt want to complicate things unnecessarily for now. > > >>Minor style request: I like that you're accessing ->on_cpu through >>functions so the !SMP case doesn't clutter the code with ifdefs... but >>can you do set_task_on_cpu(p) and clear_task_on_cpu(p) ? > > > yeah, i thought about these two variants and went for set_task_on_cpu() > so that it's less encapsulated (it's really just a conditional > assignment) and that it parallels set_task_cpu() use. But no strong > feelings either way. Anyway, lets try what we have now, i'll do the rest > in deltas. > Sounds good. -- SUSE Labs, Novell Inc.