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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id B4713C433EF for ; Mon, 13 Jun 2022 09:47:38 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S239984AbiFMJrg (ORCPT ); Mon, 13 Jun 2022 05:47:36 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:41936 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S239330AbiFMJrd (ORCPT ); Mon, 13 Jun 2022 05:47:33 -0400 Received: from mail-lj1-x22c.google.com (mail-lj1-x22c.google.com [IPv6:2a00:1450:4864:20::22c]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id BC42C17E0D; Mon, 13 Jun 2022 02:47:28 -0700 (PDT) Received: by mail-lj1-x22c.google.com with SMTP id g2so5558450ljk.5; Mon, 13 Jun 2022 02:47:28 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20210112; h=from:date:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to; bh=ei/MAvv0B3gDXqpc/eelqxW8j2ocxW4IcrNXCdHzmoc=; b=nLtgU8NhLHMO0esraVqskYzuHwgdIbAedhpn0gBuCDAMYv6RrCVwov5K1Rb76fkM91 +IUWu7X9iZn0OFM7GXHTUT493v+KeBR5dkdr2P3Isuw7VjAVqE7uMSRxrUk7nQq5lHHC 15lNx9pkycYpyjSYczY2Mx4AedEbGanBnjWUNv+KCkVSkkqHbl2MM9mmP+AuMSYiUvtJ 9lEcuoKNbPKJEjWEpjTyzXQGSPKsyw4NT1mxK+WuFgnLaF4Pd1FfQSSArgnu8FpAQ7CW vWFPUdV2K2bdF5aq0XkIOsA0BpMUAw+hco0CRHryeCgOk16PU+SD1TTh8s784yKVeZ9+ 6PcA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:from:date:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to; bh=ei/MAvv0B3gDXqpc/eelqxW8j2ocxW4IcrNXCdHzmoc=; b=wk4xPstpzSDUUFeHoxsOmfIf/9hmHf2+Nles2v/kgl4WK2a/4SdP7fiAFmKGLc35u3 Sj/ATKE4ajwh229QwrFsn/2f9wFD55ZfixaQtYuz98/L6aRWzUPG8klGGfFaMKmNcnhm R9OF5IeM6O1DhkY8aiyuiivm8bOCJVvxbMM24XRCJgXfHesfRJ6+OyI0OxVWTlrs30Vt vYglUTtOLQXm4AtE2Z1wW8Mih/B9oIP+GAKjYyEUh0YyDGGEAgxMpC+7omAhj2iGE3Hb a9eettgwzfYYJRUHEJs15uT6H/lDUlonWmmYqPhZBKbEs+V1WouLqEvMdNuTJaWlA6sS MmRg== X-Gm-Message-State: AOAM530QblfK3RFQqjPRF8qqBQy+ug0FUybjsEJw8+eRtFqEFg4af3VI wFI8i2Vk1BAp3hI+JQ719xM= X-Google-Smtp-Source: ABdhPJzyR3KFaZinkQleKOAwDelKko+1gehs2RlAAHdpUf4tAGOqz9KddIXsOmxwafkLLWranQUugg== X-Received: by 2002:a05:651c:b10:b0:255:7bfa:6b5d with SMTP id b16-20020a05651c0b1000b002557bfa6b5dmr26703808ljr.226.1655113646865; Mon, 13 Jun 2022 02:47:26 -0700 (PDT) Received: from pc638.lan ([155.137.26.201]) by smtp.gmail.com with ESMTPSA id e27-20020a19691b000000b00479025bafb5sm920519lfc.301.2022.06.13.02.47.25 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 13 Jun 2022 02:47:26 -0700 (PDT) From: Uladzislau Rezki X-Google-Original-From: Uladzislau Rezki Date: Mon, 13 Jun 2022 11:47:24 +0200 To: Joel Fernandes , "Paul E. McKenney" Cc: Uladzislau Rezki , "Paul E. McKenney" , LKML , RCU , Frederic Weisbecker , Neeraj Upadhyay , Oleksiy Avramchenko Subject: Re: [PATCH 2/2] rcu/kvfree: Introduce KFREE_DRAIN_JIFFIES_[MAX/MIN] interval Message-ID: References: <20220602080644.432156-1-urezki@gmail.com> <20220602080644.432156-2-urezki@gmail.com> <20220604155108.GU1790663@paulmck-ThinkPad-P17-Gen-1> <20220607034710.GE1790663@paulmck-ThinkPad-P17-Gen-1> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hello, Joel, Paul. > Hi Vlad, Paul, > > On Thu, Jun 09, 2022 at 03:10:57PM +0200, Uladzislau Rezki wrote: > > On Tue, Jun 7, 2022 at 5:47 AM Paul E. McKenney wrote: > > > > > > On Sun, Jun 05, 2022 at 11:10:31AM +0200, Uladzislau Rezki wrote: > > > > > On Thu, Jun 02, 2022 at 10:06:44AM +0200, Uladzislau Rezki (Sony) wrote: > > > > > > Currently the monitor work is scheduled with a fixed interval that > > > > > > is HZ/20 or each 50 milliseconds. The drawback of such approach is > > > > > > a low utilization of page slot in some scenarios. The page can store > > > > > > up to 512 records. For example on Android system it can look like: > > > > > > > > > > > > > > > > > > kworker/3:0-13872 [003] .... 11286.007048: rcu_invoke_kfree_bulk_callback: rcu_preempt bulk=0x0000000026522604 nr_records=1 > > > > > > kworker/3:0-13872 [003] .... 11286.015638: rcu_invoke_kfree_bulk_callback: rcu_preempt bulk=0x0000000095ed6fca nr_records=2 > > > > > > kworker/1:2-20434 [001] .... 11286.051230: rcu_invoke_kfree_bulk_callback: rcu_preempt bulk=0x0000000044872ffd nr_records=1 > > > > > > kworker/1:2-20434 [001] .... 11286.059322: rcu_invoke_kfree_bulk_callback: rcu_preempt bulk=0x0000000026522604 nr_records=2 > > > > > > kworker/0:1-20052 [000] .... 11286.095295: rcu_invoke_kfree_bulk_callback: rcu_preempt bulk=0x0000000044872ffd nr_records=2 > > > > > > kworker/0:1-20052 [000] .... 11286.103418: rcu_invoke_kfree_bulk_callback: rcu_preempt bulk=0x00000000cbcf05db nr_records=1 > > > > > > kworker/2:3-14372 [002] .... 11286.135155: rcu_invoke_kfree_bulk_callback: rcu_preempt bulk=0x0000000095ed6fca nr_records=2 > > > > > > kworker/2:3-14372 [002] .... 11286.135198: rcu_invoke_kfree_bulk_callback: rcu_preempt bulk=0x0000000044872ffd nr_records=1 > > > > > > kworker/1:2-20434 [001] .... 11286.155377: rcu_invoke_kfree_bulk_callback: rcu_preempt bulk=0x00000000cbcf05db nr_records=5 > > > > > > kworker/2:3-14372 [002] .... 11286.167181: rcu_invoke_kfree_bulk_callback: rcu_preempt bulk=0x0000000026522604 nr_records=5 > > > > > > kworker/1:2-20434 [001] .... 11286.179202: rcu_invoke_kfree_bulk_callback: rcu_preempt bulk=0x000000008ef95e14 nr_records=1 > > > > > > kworker/2:3-14372 [002] .... 11286.187398: rcu_invoke_kfree_bulk_callback: rcu_preempt bulk=0x00000000c597d297 nr_records=6 > > > > > > kworker/3:0-13872 [003] .... 11286.187445: rcu_invoke_kfree_bulk_callback: rcu_preempt bulk=0x0000000050bf92e2 nr_records=3 > > > > > > kworker/1:2-20434 [001] .... 11286.198975: rcu_invoke_kfree_bulk_callback: rcu_preempt bulk=0x00000000cbcf05db nr_records=4 > > > > > > kworker/1:2-20434 [001] .... 11286.207203: rcu_invoke_kfree_bulk_callback: rcu_preempt bulk=0x0000000095ed6fca nr_records=4 > > > > > > > > > > > > > > > > > > where a page only carries few records to reclaim a memory. In order to > > > > > > improve batching and make utilization more efficient the patch introduces > > > > > > a drain interval that can be set either to KFREE_DRAIN_JIFFIES_MAX or > > > > > > KFREE_DRAIN_JIFFIES_MIN. It is adjusted if a flood is detected, in this > > > > > > case a memory reclaim occurs more often whereas in mostly idle cases the > > > > > > interval is set to its maximum timeout that improves the utilization of > > > > > > page slots. > > > > > > > > > > > > Signed-off-by: Uladzislau Rezki (Sony) > > > > > > > > > > That does look like a problem well worth solving! > > > > > > > > > Agree, better ideas make better final solution :) > > > > > > > > > > > > > > But I am missing one thing. If we are having a callback flood, why do we > > > > > need a shorter timeout? > > > > > > > > > To offload faster, because otherwise we run into classical issue, it is a low > > > > memory condition state resulting in OOM. > > > > > > But doesn't each callback queued during the flood give us an opportunity > > > to react to the flood? That will be way more fine-grained than any > > > reasonable timer, right? Or am I missing something? > > > > > We can set the timer to zero or to current "jiffies" to initiate the > > offloading if the > > page is full. In that sense probably it make sense to propagate those two attr. > > to user space, so the user can configure min/max drain interval. > > > > Or we can only deal with fixed interval exposed via sysfs to control it by user. > > In that case we can get rid of MIN one and just trigger a timer if the page is > > full. I think this approach is better. > > Yes I also think triggering timer with zero-timeout is better. Can you (Vlad) > accomplish that by just calling the timer callback inline, instead of queuing > a timer? I imagine you would just do queue_work() instead of > queue_delayed_work() in this scenario. > > > > I do agree that the action would often need to be indirect to avoid the > > > memory-allocation-state hassles, but we already can do that, either via > > > an extremely short-term hrtimer or something like irq-work. > > > > > > > > Wouldn't a check on the number of blocks queued be simpler, more direct, > > > > > and provide faster response to the start of a callback flood? > > > > > > > > > I rely on krcp->count because not always we can store the pointer in the page > > > > slots. We can not allocate a page in the caller context thus we use page-cache > > > > worker that fills the cache in normal context. While it populates the cache, > > > > pointers temporary are queued to the linked-list. > > > > > > > > Any thoughts? > > > > > > There are a great many ways to approach this. One of them is to maintain > > > a per-CPU free-running counter of kvfree_rcu() calls, and to reset this > > > counter each jiffy. > > > > > > Or am I missing a trick here? > > > > > Do you mean to have a per-cpu timer that checks the per-cpu-freed counter > > and schedule the work when if it is needed? Or i have missed your point? > > I think he (Paul) is describing the way 'flood detection' can work similar to how the > bypass list code is implemented. There he maintains a count which only if > exceeds a limit, will queue on to the bypass list. > OK, i see that. We also do similar thing. We say it is a flood - when a page becomes full, so it is kind of threshold that we pass. > This code: > > // If we have advanced to a new jiffy, reset counts to allow > // moving back from ->nocb_bypass to ->cblist. > if (j == rdp->nocb_nobypass_last) { > c = rdp->nocb_nobypass_count + 1; > } else { > WRITE_ONCE(rdp->nocb_nobypass_last, j); > c = rdp->nocb_nobypass_count - nocb_nobypass_lim_per_jiffy; > if (ULONG_CMP_LT(rdp->nocb_nobypass_count, > nocb_nobypass_lim_per_jiffy)) > c = 0; > else if (c > nocb_nobypass_lim_per_jiffy) > c = nocb_nobypass_lim_per_jiffy; > } > WRITE_ONCE(rdp->nocb_nobypass_count, c); > > > Your (Vlad's) approach OTOH is also fine to me, you check if page is full and > make that as a 'flood is happening' detector. > OK, thank you Joel. I also think, that way we improve batching and utilization of the page what is actually an intention of the patch in question. -- Uladzislau Rezki