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=-8.0 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,FREEMAIL_FORGED_FROMDOMAIN,FREEMAIL_FROM, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS 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 E8348C07E99 for ; Fri, 9 Jul 2021 13:47:46 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id C701061380 for ; Fri, 9 Jul 2021 13:47:46 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231622AbhGINu3 (ORCPT ); Fri, 9 Jul 2021 09:50:29 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:50774 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231572AbhGINu3 (ORCPT ); Fri, 9 Jul 2021 09:50:29 -0400 Received: from mail-qk1-x72b.google.com (mail-qk1-x72b.google.com [IPv6:2607:f8b0:4864:20::72b]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id A127DC0613DD for ; Fri, 9 Jul 2021 06:47:45 -0700 (PDT) Received: by mail-qk1-x72b.google.com with SMTP id b18so9326490qkc.5 for ; Fri, 09 Jul 2021 06:47:45 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to; bh=EQUeRKfr59EhNaXqx/238AuLWVll4VFVIL4GFqoBf7k=; b=iVBTjPuD4qf+tTpCOK1WoaFE8JKIXoxmtXGEm9P2QY/5u9oOxiKVghtaOdrsqyizoj WwgjqzYs+zvh8Q12D/YqbDTRWwQGLMdg2/CbWxBFAXuw7ggLvBqlo3x88AdqWIISY0Gg 9Ki3GfKV+i8Dr0nwW+BOBxAhKdyDwMTMJVvVPVxgVbAzlkqZ2Q4y0KZv7plS65SInuTQ Xa9QQ7cHz3gHWh440YAU2jto4SR3Y4eyIM6AbblqG72CG2E0/jkb3jya1KaAnGgu11x9 nnd47EqOdmkty2RXbcrdZ/XXJp/k7teJ4ihtLeVH2qZtEyf6C1k4l1iX/umByjEpsHKq 0U3w== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to; bh=EQUeRKfr59EhNaXqx/238AuLWVll4VFVIL4GFqoBf7k=; b=MCbGGFMjUAuDjBEQN6RvIOytyT5i/3FQTe1x/QK+Ouf/bPRPMBoEZy+knu2Q05gIb1 xtC/gZDKg3vNmpcB+4hRq3nYHqO8YYBeKINmPKPe/HcH7SWZlUGlZSprjCNuX43F3Ib8 D9X3/q4Amc6b4czYgwU/o3v0x1ptQoWE0lTQXvq+qIqJ2xB0T7468B+vAPD4DCnotDSx tP/aLOABGHChsXce0lCoUb/CwIr+X9qgYWf/3fIfyOiIczcGaRfMO2cWeQRMyhiFHvxl l/A7O7WAZZTGsN5DASTmBsfD63l0ImtAVKazd0dIl0tOaM6lB5TZ6NNIJSjZX+SLFLI+ OxJw== X-Gm-Message-State: AOAM5308VZmvk3sPNV4F/04Pt3xktNi0bL+EKCl86Z7ave2lDP5ey06s y65HT7Q76bctoYZqQewDZRA= X-Google-Smtp-Source: ABdhPJz9Rre5TNQRA0YQZHDJhxc+uRithR40Lfww3HewpS+oi0CDB2Jr2LRLOkqBQVOE1mXWgjvZEw== X-Received: by 2002:a05:620a:1233:: with SMTP id v19mr764840qkj.33.1625838464879; Fri, 09 Jul 2021 06:47:44 -0700 (PDT) Received: from dschatzberg-fedora-PC0Y6AEN ([2620:10d:c091:480::1:48a0]) by smtp.gmail.com with ESMTPSA id l206sm2436503qke.80.2021.07.09.06.47.43 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 09 Jul 2021 06:47:44 -0700 (PDT) Date: Fri, 9 Jul 2021 09:47:42 -0400 From: Dan Schatzberg To: Ming Lei Cc: Jens Axboe , linux-block@vger.kernel.org, Christoph Hellwig , Michal =?iso-8859-1?Q?Koutn=FD?= Subject: Re: [PATCH 6/6] loop: don't add worker into idle list Message-ID: References: <20210705102607.127810-1-ming.lei@redhat.com> <20210705102607.127810-7-ming.lei@redhat.com> 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-block@vger.kernel.org On Fri, Jul 09, 2021 at 08:49:55AM +0800, Ming Lei wrote: > On Thu, Jul 08, 2021 at 11:15:13AM -0400, Dan Schatzberg wrote: > > On Thu, Jul 08, 2021 at 11:01:54PM +0800, Ming Lei wrote: > > > On Thu, Jul 08, 2021 at 10:16:50AM -0400, Dan Schatzberg wrote: > > > > On Thu, Jul 08, 2021 at 02:58:36PM +0800, Ming Lei wrote: > > > > > On Wed, Jul 07, 2021 at 09:55:34AM -0400, Dan Schatzberg wrote: > > > > > > On Wed, Jul 07, 2021 at 11:19:14AM +0800, Ming Lei wrote: > > > > > > > On Tue, Jul 06, 2021 at 09:55:36AM -0400, Dan Schatzberg wrote: > > > > > > > > On Mon, Jul 05, 2021 at 06:26:07PM +0800, Ming Lei wrote: > > > > > > > > > } > > > > > > > > > + > > > > > > > > > + spin_lock(lock); > > > > > > > > > list_add_tail(&cmd->list_entry, cmd_list); > > > > > > > > > + spin_unlock(lock); > > > > > > > > > queue_work(lo->workqueue, work); > > > > > > > > > - spin_unlock(&lo->lo_work_lock); > > > > > > > > > } > > > > > > > > > > > > > > > > > > static void loop_update_rotational(struct loop_device *lo) > > > > > > > > > @@ -1131,20 +1159,18 @@ static void loop_set_timer(struct loop_device *lo) > > > > > > > > > > > > > > > > > > static void __loop_free_idle_workers(struct loop_device *lo, bool force) > > > > > > > > > { > > > > > > > > > - struct loop_worker *pos, *worker; > > > > > > > > > + struct loop_worker *worker; > > > > > > > > > + unsigned long id; > > > > > > > > > > > > > > > > > > spin_lock(&lo->lo_work_lock); > > > > > > > > > - list_for_each_entry_safe(worker, pos, &lo->idle_worker_list, > > > > > > > > > - idle_list) { > > > > > > > > > + xa_for_each(&lo->workers, id, worker) { > > > > > > > > > if (!force && time_is_after_jiffies(worker->last_ran_at + > > > > > > > > > LOOP_IDLE_WORKER_TIMEOUT)) > > > > > > > > > break; > > > > > > > > > - list_del(&worker->idle_list); > > > > > > > > > - xa_erase(&lo->workers, worker->blkcg_css->id); > > > > > > > > > - css_put(worker->blkcg_css); > > > > > > > > > - kfree(worker); > > > > > > > > > + if (refcount_dec_and_test(&worker->refcnt)) > > > > > > > > > + loop_release_worker(worker); > > > > > > > > > > > > > > > > This one is puzzling to me. Can't you hit this refcount decrement > > > > > > > > superfluously each time the loop timer fires? > > > > > > > > > > > > > > Not sure I get your point. > > > > > > > > > > > > > > As I mentioned above, this one is the counter pair of INIT reference, > > > > > > > but one new lo_cmd may just grab it when queueing rq before erasing the > > > > > > > worker from xarray, so we can't release worker here until the command is > > > > > > > completed. > > > > > > > > > > > > Suppose at this point there's still an outstanding loop_cmd to be > > > > > > serviced for this worker. The refcount_dec_and_test should decrement > > > > > > the refcount and then fail the conditional, not calling > > > > > > loop_release_worker. What happens if __loop_free_idle_workers fires > > > > > > again before the loop_cmd is processed? Won't you decrement the > > > > > > refcount again, and then end up calling loop_release_worker before the > > > > > > loop_cmd is processed? > > > > > > > > > > Good catch! > > > > > > > > > > The following one line change should avoid the issue: > > > > > > > > > > diff --git a/drivers/block/loop.c b/drivers/block/loop.c > > > > > index 146eaa03629b..3cd51bddfec9 100644 > > > > > --- a/drivers/block/loop.c > > > > > +++ b/drivers/block/loop.c > > > > > @@ -980,7 +980,6 @@ static struct loop_worker *loop_alloc_or_get_worker(struct loop_device *lo, > > > > > > > > > > static void loop_release_worker(struct loop_worker *worker) > > > > > { > > > > > - xa_erase(&worker->lo->workers, worker->blkcg_css->id); > > > > > css_put(worker->blkcg_css); > > > > > kfree(worker); > > > > > } > > > > > @@ -1167,6 +1166,7 @@ static void __loop_free_idle_workers(struct loop_device *lo, bool force) > > > > > if (!force && time_is_after_jiffies(worker->last_ran_at + > > > > > LOOP_IDLE_WORKER_TIMEOUT)) > > > > > break; > > > > > + xa_erase(&worker->lo->workers, worker->blkcg_css->id); > > > > > if (refcount_dec_and_test(&worker->refcnt)) > > > > > loop_release_worker(worker); > > > > > } > > > > > > > > Yeah, I think this resolves the issue. You could end up repeatedly > > > > allocating workers for the same blkcg in the event that you're keeping > > > > the worker busy for the entire LOOP_IDLE_WORKER_TIMEOUT (since it only > > > > updates the last_ran_at when idle). You may want to add a racy check > > > > if the refcount is > 1 to avoid that. > > > > > > Given the event is very unlikely to trigger, I think we can live > > > with that. > > > > It doesn't seem unlikely to me - any workload that saturates the > > backing device would keep the loop worker constantly with at least one > > loop_cmd queued and trigger a free and allocate every > > LOOP_IDLE_WORKER_TIMEOUT. Another way to solve this is to just update > > last_ran_at before or after each loop_cmd. In any event, I'll defer to > > your decision, it's not a critical difference. > > Sorry, I missed that ->last_ran_at is only set when the work isn't > pending, then we can cleanup/simplify the reclaim a bit by: > > 1) keep lo->idle_work to be scheduled in 60 period if there is any > active worker allocated, which is scheduled when allocating/reclaiming > one worker Makes sense, and you should have lo_work_lock held at both points so this is safe. > > 2) always set ->last_ran_at after retrieving the worker from xarray, > which can be done lockless via WRITE_ONCE(), and it is cheap Yes, or in loop_process_work, doesn't really matter where you do it so long as it is per-cmd. I think this change alone resolves the issue. > > 3) inside __loop_free_idle_workers(), reclaim one worker only if the > worker is expired and hasn't commands in worker->cmd_list Be careful here - the current locking doesn't allow for this because you don't acquire the per-worker lock in __loop_free_idle_workers, so accessing worker->cmd_list is a data-race. This is why I suggested reading the refcount instead as it can be done without holding a lock. > > > > > > > > > > > > > > I think there might be a separate issue with the locking here though - > > > > you acquire the lo->lo_work_lock in __loop_free_idle_workers and then > > > > check worker->last_ran_at for each worker. However you only protect > > > > the write to worker->last_ran_at (in loop_process_work) with the > > > > worker->lock which I think means there's a potential data race on > > > > worker->last_ran_at. > > > > > > It should be fine since both WRITE and READ on worker->last_ran_at is > > > atomic. Even though the race is triggered, we still can live with that. > > > > True, though in this case I think last_ran_at should be atomic_t with > > atomic_set and atomic_read. > > I think READ_ONCE()/WRITE_ONCE() should be enough, and we can set/get > last_ran_at lockless. Makes sense to me