From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pb0-f46.google.com (mail-pb0-f46.google.com [209.85.160.46]) by kanga.kvack.org (Postfix) with ESMTP id E42CA6B0031 for ; Wed, 2 Oct 2013 08:21:02 -0400 (EDT) Received: by mail-pb0-f46.google.com with SMTP id rq2so797870pbb.19 for ; Wed, 02 Oct 2013 05:21:02 -0700 (PDT) Date: Wed, 2 Oct 2013 14:13:56 +0200 From: Oleg Nesterov Subject: Re: [PATCH] hotplug: Optimize {get,put}_online_cpus() Message-ID: <20131002121356.GA21581@redhat.com> References: <20130925175055.GA25914@redhat.com> <20130928144720.GL15690@laptop.programming.kicks-ass.net> <20130928163104.GA23352@redhat.com> <7632387.20FXkuCITr@vostro.rjw.lan> <524B0233.8070203@linux.vnet.ibm.com> <20131001173615.GW3657@laptop.programming.kicks-ass.net> <20131001174508.GA17411@redhat.com> <20131001175640.GQ15690@laptop.programming.kicks-ass.net> <20131001180750.GA18261@redhat.com> <20131002090859.GE12926@twins.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20131002090859.GE12926@twins.programming.kicks-ass.net> Sender: owner-linux-mm@kvack.org List-ID: To: Peter Zijlstra Cc: "Srivatsa S. Bhat" , "Rafael J. Wysocki" , "Paul E. McKenney" , Mel Gorman , Rik van Riel , Srikar Dronamraju , Ingo Molnar , Andrea Arcangeli , Johannes Weiner , Linux-MM , LKML , Thomas Gleixner , Steven Rostedt , Viresh Kumar On 10/02, Peter Zijlstra wrote: > > On Tue, Oct 01, 2013 at 08:07:50PM +0200, Oleg Nesterov wrote: > > > > But note that you do not strictly need this change. Just kill cpuhp_waitcount, > > > > then we can change cpu_hotplug_begin/end to use xxx_enter/exit we discuss in > > > > another thread, this should likely "join" all synchronize_sched's. > > > > > > That would still be 4k * sync_sched() == terribly long. > > > > No? the next xxx_enter() avoids sync_sched() if rcu callback is still > > pending. Unless __cpufreq_remove_dev_finish() is "too slow" of course. > > Hmm,. not in the version you posted; there xxx_enter() would only not do > the sync_sched if there's a concurrent 'writer', in which case it will > wait for it. No, please see below. > You only avoid the sync_sched in xxx_exit() and potentially join in the > sync_sched() of a next xxx_begin(). > > So with that scheme: > > for (i= ; i<4096; i++) { > xxx_begin(); > xxx_exit(); > } > > Will get 4096 sync_sched() calls from the xxx_begin() and all but the > last xxx_exit() will 'drop' the rcu callback. No, the code above should call sync_sched() only once, no matter what this code does between _enter and _exit. This was one of the points. To clarify, of course I mean the "likely" case. Say, a long preemption after _exit can lead to another sync_sched(). void xxx_enter(struct xxx_struct *xxx) { bool need_wait, need_sync; spin_lock_irq(&xxx->xxx_lock); need_wait = xxx->gp_count++; need_sync = xxx->gp_state == GP_IDLE; if (need_sync) xxx->gp_state = GP_PENDING; spin_unlock_irq(&xxx->xxx_lock); BUG_ON(need_wait && need_sync); if (need_sync) { synchronize_sched(); xxx->gp_state = GP_PASSED; wake_up_all(&xxx->gp_waitq); } else if (need_wait) { wait_event(&xxx->gp_waitq, xxx->gp_state == GP_PASSED); } else { BUG_ON(xxx->gp_state != GP_PASSED); } } The 1st iteration: xxx_enter() does synchronize_sched() and sets gp_state = GP_PASSED. xxx_exit() starts the rcu callback, but gp_state is still PASSED. all other iterations in the "likely" case: xxx_enter() should likely come before the pending callback fires and clears gp_state. In this case we only increment ->gp_count (this "disables" the rcu callback) and do nothing more, gp_state is still GP_PASSED. xxx_exit() does another call_rcu_sched(), or does the CP_PENDING -> CB_REPLAY change. The latter is the same as "start another callback". In short: unless a gp elapses between _exit() and _enter(), the next _enter() does nothing and avoids synchronize_sched(). > And given the construct; I'm not entirely sure you can do away with the > sync_sched() in between. While its clear to me you can merge the two > into one; leaving it out entirely doesn't seem right. Could you explain? Oleg. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org