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[209.85.220.65]) by mx.google.com with SMTPS id r6sor49420335wro.6.2019.01.14.11.30.24 for (Google Transport Security); Mon, 14 Jan 2019 11:30:24 -0800 (PST) Received-SPF: pass (google.com: domain of surenb@google.com designates 209.85.220.65 as permitted sender) client-ip=209.85.220.65; Authentication-Results: mx.google.com; dkim=pass header.i=@google.com header.s=20161025 header.b=LTdpmr7i; spf=pass (google.com: domain of surenb@google.com designates 209.85.220.65 as permitted sender) smtp.mailfrom=surenb@google.com; dmarc=pass (p=REJECT sp=REJECT dis=NONE) header.from=google.com DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20161025; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=V7Nld/m9dECZLpYS+KOKxjAa17WRgnctGhf+2cl141w=; b=LTdpmr7iYb+51Ff7+r2qP/Jw2u8ZRJFk77VCkLCCiA/ANUAVLSsCEHHlNFOKFkEwnD rxV7NnDpIhv7eJ+Y/9f7Iuok4tIdFaSDBWG3QChNJ8RlE2bArnmE+J4ig2eAqeCDIrA3 jjQuOc8YmNbAupjvk5iECrrfxoqfba70+VzYHLoB8hFfCP3mS0r34T7J1PWePGNK9iDh t4Gm5BhbYr8c4rOv07T+KyS8HI/q9jrOW9ijC873iQAKVsVgbcHtRN8VCyKQ8Csd3NSm OplHhfMZZn83IGt7jkFnKt2soK1fIUBMXub+E85LXFf1Mn5znpEXQqjmHtygPCW7DB2l OYKw== X-Google-Smtp-Source: ALg8bN6zwKZcThH+o+SBsAintWmrcGydROfOrohi8vkW1zna6VHepdzbVgey6gGzGvNUXUo29kRjKBoH/sqTpKlTCv4= X-Received: by 2002:adf:de91:: with SMTP id w17mr38740wrl.320.1547494224091; Mon, 14 Jan 2019 11:30:24 -0800 (PST) MIME-Version: 1.0 References: <20190110220718.261134-1-surenb@google.com> <20190110220718.261134-6-surenb@google.com> <20190114102137.GB14054@worktop.programming.kicks-ass.net> In-Reply-To: <20190114102137.GB14054@worktop.programming.kicks-ass.net> From: Suren Baghdasaryan Date: Mon, 14 Jan 2019 11:30:12 -0800 Message-ID: Subject: Re: [PATCH v2 5/5] psi: introduce psi monitor To: Peter Zijlstra Cc: Greg Kroah-Hartman , Tejun Heo , lizefan@huawei.com, Johannes Weiner , axboe@kernel.dk, dennis@kernel.org, Dennis Zhou , Ingo Molnar , Andrew Morton , Jonathan Corbet , cgroups@vger.kernel.org, linux-mm , linux-doc@vger.kernel.org, LKML , kernel-team@android.com Content-Type: text/plain; charset="UTF-8" X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: Message-ID: <20190114193012.QTg6Lp0Fs96gFV7MQLsYKyH8Tt-Y5Y323l6Qlf3F4RA@z> On Mon, Jan 14, 2019 at 2:22 AM Peter Zijlstra wrote: > > On Thu, Jan 10, 2019 at 02:07:18PM -0800, Suren Baghdasaryan wrote: > > +/* > > + * psi_update_work represents slowpath accounting part while > > + * psi_group_change represents hotpath part. > > + * There are two potential races between these path: > > + * 1. Changes to group->polling when slowpath checks for new stall, then > > + * hotpath records new stall and then slowpath resets group->polling > > + * flag. This leads to the exit from the polling mode while monitored > > + * states are still changing. > > + * 2. Slowpath overwriting an immediate update scheduled from the hotpath > > + * with a regular update further in the future and missing the > > + * immediate update. > > + * Both races are handled with a retry cycle in the slowpath: > > + * > > + * HOTPATH: | SLOWPATH: > > + * | > > + * A) times[cpu] += delta | E) delta = times[*] > > + * B) start_poll = (delta[poll_mask] &&| if delta[poll_mask]: > > + * cmpxchg(g->polling, 0, 1) == 0)| F) polling_until = now + > > + * | grace_period > > + * | if now > polling_until: > > + * if start_poll: | if g->polling: > > + * C) mod_delayed_work(1) | G) g->polling = polling = 0 > > + * else if !delayed_work_pending(): | H) goto SLOWPATH > > + * D) schedule_delayed_work(PSI_FREQ)| else: > > + * | if !g->polling: > > + * | I) g->polling = polling = 1 > > + * | J) if delta && first_pass: > > + * | next_avg = calculate_averages() > > + * | if polling: > > + * | next_poll = poll_triggers() > > + * | if (delta && first_pass) || polling: > > + * | K) mod_delayed_work( > > + * | min(next_avg, next_poll)) > > + * | if !polling: > > + * | first_pass = false > > + * | L) goto SLOWPATH > > + * > > + * Race #1 is represented by (EABGD) sequence in which case slowpath > > + * deactivates polling mode because it misses new monitored stall and hotpath > > + * doesn't activate it because at (B) g->polling is not yet reset by slowpath > > + * in (G). This race is handled by the (H) retry, which in the race described > > + * above results in the new sequence of (EABGDHEIK) that reactivates polling > > + * mode. > > + * > > + * Race #2 is represented by polling==false && (JABCK) sequence which > > + * overwrites immediate update scheduled at (C) with a later (next_avg) update > > + * scheduled at (K). This race is handled by the (L) retry which results in the > > + * new sequence of polling==false && (JABCKLEIK) that reactivates polling mode > > + * and reschedules next polling update (next_poll). > > + * > > + * Note that retries can't result in an infinite loop because retry #1 happens > > + * only during polling reactivation and retry #2 happens only on the first > > + * pass. Constant reactivations are impossible because polling will stay active > > + * for at least grace_period. Worst case scenario involves two retries (HEJKLE) > > + */ > > I'm having a fairly hard time with this. There's a distinct lack of > memory ordering, and a suspicious mixing of atomic ops (cmpxchg) and > regular loads and stores (without READ_ONCE/WRITE_ONCE even). > > Please clarify. Thanks for the feedback. I do mix atomic and regular loads with g->polling only because the slowpath is the only one that resets it back to 0, so cmpxchg(g->polling, 1, 0) == 1 at (G) would always return 1. Setting g->polling back to 1 at (I) indeed needs an atomic operation but at that point it does not matter whether hotpath or slowpath sets it. In either case we will schedule a polling update. Am I missing anything? For memory ordering (which Johannes also pointed out) the critical point is: times[cpu] += delta | if g->polling: smp_wmb() | g->polling = polling = 0 cmpxchg(g->polling, 0, 1) | smp_rmb() | delta = times[*] (through goto SLOWPATH) So that hotpath writes to times[] then g->polling and slowpath reads g->polling then times[]. cmpxchg() implies a full barrier, so we can drop smp_wmb(). Something like this: times[cpu] += delta | if g->polling: cmpxchg(g->polling, 0, 1) | g->polling = polling = 0 | smp_rmb() | delta = times[*] (through goto SLOWPATH) Would that address your concern about ordering? > (also, you look to have a whole bunch of line-breaks that are really not > needed; concattenated the line would not be over 80 chars). Will try to minimize line-breaks. > -- > You received this message because you are subscribed to the Google Groups "kernel-team" group. > To unsubscribe from this group and stop receiving emails from it, send an email to kernel-team+unsubscribe@android.com. >