From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-11.4 required=3.0 tests=BAYES_00,DKIMWL_WL_MED, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_IN_DEF_DKIM_WL autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id F24C6C433E7 for ; Mon, 12 Oct 2020 14:53:15 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 925C92078E for ; Mon, 12 Oct 2020 14:53:15 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b="ckuY1EM3" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 925C92078E Authentication-Results: mail.kernel.org; dmarc=fail (p=reject dis=none) header.from=google.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id BCDC594000A; Mon, 12 Oct 2020 10:53:14 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id B7F70900002; Mon, 12 Oct 2020 10:53:14 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id A205E94000A; Mon, 12 Oct 2020 10:53:14 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0021.hostedemail.com [216.40.44.21]) by kanga.kvack.org (Postfix) with ESMTP id 72614900002 for ; Mon, 12 Oct 2020 10:53:14 -0400 (EDT) Received: from smtpin03.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay01.hostedemail.com (Postfix) with ESMTP id 0DF79180AD806 for ; Mon, 12 Oct 2020 14:53:14 +0000 (UTC) X-FDA: 77363566308.03.ink25_410b201271fb Received: from filter.hostedemail.com (10.5.16.251.rfc1918.com [10.5.16.251]) by smtpin03.hostedemail.com (Postfix) with ESMTP id DE04028A4E9 for ; Mon, 12 Oct 2020 14:53:13 +0000 (UTC) X-HE-Tag: ink25_410b201271fb X-Filterd-Recvd-Size: 4422 Received: from mail-ej1-f66.google.com (mail-ej1-f66.google.com [209.85.218.66]) by imf42.hostedemail.com (Postfix) with ESMTP for ; Mon, 12 Oct 2020 14:53:13 +0000 (UTC) Received: by mail-ej1-f66.google.com with SMTP id qp15so23591287ejb.3 for ; Mon, 12 Oct 2020 07:53:13 -0700 (PDT) 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=han/y2ObDrzU2Lpo5dfsQQuUnJc22R+SVZVH0ama1LA=; b=ckuY1EM3qdBtaRKpMwYzHlt+WxEztI2VtQk77OwdIlLvq7sXhvdHFoxvCOfd1ucivR iY2t4tGcjK2+1eSXNTHL8fBgDSE1tQYu9cHSTqAPjQmtuNXXn16cOwmw1uHIo49cxeSE 6HFZJEe+wA40V62J6qG/44/2Vt7Dtt4K1LHaQe35lxzqqauRJFoegn9HRK/tU61tOu6j iP90XPesD6khF0CvbR/iP9QVwwymLVbhUznHnYgsvTX2oUn28AkoB6u0Iu1hmf5gQSNr 6tFB4xy15BN5AnMkqvQbAtJpuGVHOKuRiA3tL7em2O9/J23chJxL/bszoY35yx+rNziI 4MWQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=han/y2ObDrzU2Lpo5dfsQQuUnJc22R+SVZVH0ama1LA=; b=gUQ2NBd3Psb0up3PvaGVWGgJv4YSzpFW4LrHiX4sA1zL5h+C7wyBSf1Qu3CjIHeOTY OApPeng9ce02xNRintz925Ld4Vdh1QfZYhALNMOxIU5Kw4tEgY/4YijWHRtRqlpvwxjN CRdlWJuPMqa/0sTnq7X7+pi02ruW7BijYcHytaL0ifIQ9UjNBx6mjdrXNcEyQzypQ95U l9asoO7IVG5CoTEfpI0eFMd4edkgEUwHHp7s64toOu7WFGxJqFq1KGXNpm9m/Vbq6ouQ jGoFD5PE9vYqz36hM3XIVpvApMn4kCrNhfPI/Rf9T+hdx+nN6sNUCzWHlqplpguVkN+O nBKg== X-Gm-Message-State: AOAM530dh2UK9zdIxaCwlyd4g+9HkO/EHG7p1B5I/dTmUbmZZRdN6aE+ A8ew9Jo03RbPUaypIfipJ5HsJ1nWvPvrKDbI94PLWE6hFZod8g== X-Google-Smtp-Source: ABdhPJwjXVHDIhhac+d1A+3GEjWkFivZDxEV6CgWjvNihJmLoJ8H0vky1VcauI2m5vli/zH3j4rMnpv+M1t03d09H+s= X-Received: by 2002:a17:906:c0d1:: with SMTP id bn17mr17139876ejb.114.1602514391843; Mon, 12 Oct 2020 07:53:11 -0700 (PDT) MIME-Version: 1.0 References: <20201012114940.1317510-1-jannh@google.com> In-Reply-To: <20201012114940.1317510-1-jannh@google.com> From: Jann Horn Date: Mon, 12 Oct 2020 16:52:45 +0200 Message-ID: Subject: Re: [PATCH] proc.5: Document inaccurate RSS due to SPLIT_RSS_COUNTING To: Michael Kerrisk-manpages Cc: linux-man , Linux-MM , Mark Mossberg 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: On Mon, Oct 12, 2020 at 1:49 PM Jann Horn wrote: > Since 34e55232e59f7b19050267a05ff1226e5cd122a5 (introduced back in > v2.6.34), Linux uses per-thread RSS counters to reduce cache contention on > the per-mm counters. With a 4K page size, that means that you can end up > with the counters off by up to 252KiB per thread. Actually, as Mark Mossberg pointed out to me off-thread, the counters can actually be off by many times more... can be reproduced with e.g. the following: #include #include #include #include #include #include #include #include void dump(int pid) { char cmd[1000]; sprintf(cmd, "grep '^VmRSS' /proc/%d/status;" "grep '^Rss:' /proc/%d/smaps_rollup;" "echo", pid, pid ); system(cmd); } int main(void) { eventfd_t dummy; int child_wait = eventfd(0, EFD_SEMAPHORE|EFD_CLOEXEC); int child_resume = eventfd(0, EFD_SEMAPHORE|EFD_CLOEXEC); if (child_wait == -1 || child_resume == -1) err(1, "eventfd"); pid_t child = fork(); if (child == -1) err(1, "fork"); if (child == 0) { if (prctl(PR_SET_PDEATHSIG, SIGKILL)) err(1, "PDEATHSIG"); if (getppid() == 1) exit(0); char *mapping = mmap(NULL, 80 * 0x1000, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0); for (int i=0; 1; i++) { eventfd_write(child_wait, 1); eventfd_read(child_resume, &dummy); if (i == 80) break; mapping[0x1000 * i] = 1; } exit(0); } for (int i=0; i<81; i++) { eventfd_read(child_wait, &dummy); dump(child); eventfd_write(child_resume, 1); } exit(0); } I'm not entirely sure why though.