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Message-ID: <20150716054626.GV19282@twins.programming.kicks-ass.net> Date: Thu, 16 Jul 2015 07:46:26 +0200 From: Peter Zijlstra <peterz@...radead.org> To: Waiman Long <waiman.long@...com> Cc: Ingo Molnar <mingo@...hat.com>, Thomas Gleixner <tglx@...utronix.de>, "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org, linux-kernel@...r.kernel.org, Scott J Norton <scott.norton@...com>, Douglas Hatch <doug.hatch@...com>, Davidlohr Bueso <dave@...olabs.net> Subject: Re: [PATCH v2 4/6] locking/pvqspinlock: Allow vCPUs kick-ahead On Wed, Jul 15, 2015 at 10:01:02PM -0400, Waiman Long wrote: > On 07/15/2015 05:39 AM, Peter Zijlstra wrote: > >On Tue, Jul 14, 2015 at 10:13:35PM -0400, Waiman Long wrote: > >>Frequent CPU halting (vmexit) and CPU kicking (vmenter) lengthens > >>critical section and block forward progress. This patch implements > >>a kick-ahead mechanism where the unlocker will kick the queue head > >>vCPUs as well as up to four additional vCPUs next to the queue head > >>if they were halted. The kickings are done after exiting the critical > >>section to improve parallelism. > >> > >>The amount of kick-ahead allowed depends on the number of vCPUs > >>in the VM guest. This patch, by itself, won't do much as most of > >>the kickings are currently done at lock time. Coupled with the next > >>patch that defers lock time kicking to unlock time, it should improve > >>overall system performance in a busy overcommitted guest. > >> > >>Linux kernel builds were run in KVM guest on an 8-socket, 4 > >>cores/socket Westmere-EX system and a 4-socket, 8 cores/socket > >>Haswell-EX system. Both systems are configured to have 32 physical > >>CPUs. The kernel build times before and after the patch were: > >> > >> Westmere Haswell > >> Patch 32 vCPUs 48 vCPUs 32 vCPUs 48 vCPUs > >> ----- -------- -------- -------- -------- > >> Before patch 3m25.0s 10m34.1s 2m02.0s 15m35.9s > >> After patch 3m27.4s 10m32.0s 2m00.8s 14m52.5s > >> > >>There wasn't too much difference before and after the patch. > >That means either the patch isn't worth it, or as you seem to imply its > >in the wrong place in this series. > > It needs to be coupled with the next patch to be effective as most of the > kicking are happening at the lock side, instead of at the unlock side. If > you look at the sample pvqspinlock stats in patch 3: > > lock_kick_count=755354 > unlock_kick_count=87 > > The number of unlock kicks is negligible compared with the lock kicks. Patch > 5 does have a dependency on patch 4 unless we make it unconditionally defers > kicking to the unlock call which was what I had done in the v1 patch. The > reason why I change this in v2 is because I found a very slight performance > degradation in doing so. This way we cannot see the gains of the proposed complexity. So put it in a place where you can. > >You also do not offer any support for any of the magic numbers.. > > I chose 4 for PV_KICK_AHEAD_MAX as I didn't see much performance difference > when I did a kick-ahead of 5. Also, it may be too unfair to the vCPU that > was doing the kicking if the number is too big. Another magic number is > pv_kick_ahead number. This one is kind of arbitrary. Right now I do a log2, > but it can be divided by 4 (rshift 2) as well. So what was the difference between 1-2-3-4 ? I would be thinking one extra kick is the biggest help, no? -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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