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 64083C4167D for ; Tue, 19 Apr 2022 16:47:08 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1355424AbiDSQtr (ORCPT ); Tue, 19 Apr 2022 12:49:47 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:54868 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1349998AbiDSQpV (ORCPT ); Tue, 19 Apr 2022 12:45:21 -0400 Received: from out1.migadu.com (out1.migadu.com [IPv6:2001:41d0:2:863f::]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 91C0933E3A for ; Tue, 19 Apr 2022 09:42:38 -0700 (PDT) Date: Tue, 19 Apr 2022 09:42:30 -0700 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1650386556; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=BgIKdz66kPcyA341qp0ZDvtyQNkSL0ly49y8sDVGu0o=; b=Sef1m0iG1d4B3iQ18wyIK/tdzdNhUvFwVR4i52oZGdDUaUUm/8B5Y0mg2tchnjcLdkQ4t2 Vzl+P/VZv4cKLNJNimLr6Q9eq/SzlrjcFppI13fiIcPXI2P8Mk4F+awJPlmV0GBOHomtAl /+5Rvql1mr/GXTF4nikqR5Z42XJNNrc= X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Roman Gushchin To: David Hildenbrand Cc: linux-mm@kvack.org, Andrew Morton , Dave Chinner , linux-kernel@vger.kernel.org, Johannes Weiner , Michal Hocko , Shakeel Butt , Yang Shi Subject: Re: [PATCH] mm: do not call add_nr_deferred() with zero deferred Message-ID: References: <20220416004104.4089743-1-roman.gushchin@linux.dev> <59404249-de0c-c569-d04a-9da38ed14b0a@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <59404249-de0c-c569-d04a-9da38ed14b0a@redhat.com> X-Migadu-Flow: FLOW_OUT X-Migadu-Auth-User: linux.dev Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Apr 19, 2022 at 02:56:06PM +0200, David Hildenbrand wrote: > On 16.04.22 02:41, Roman Gushchin wrote: > > add_nr_deferred() is often called with next_deferred equal to 0. > > For instance, it's happening under low memory pressure for any > > shrinkers with a low number of cached objects. A corresponding trace > > looks like: > > <...>-619914 [005] .... 467456.345160: mm_shrink_slab_end: \ > > super_cache_scan+0x0/0x1a0 0000000087027f06: nid: 1 \ > > unused scan count 0 new scan count 0 total_scan 0 \ > > last shrinker return val 0 > > > > <...>-619914 [005] .... 467456.345371: mm_shrink_slab_end: \ > > super_cache_scan+0x0/0x1a0 0000000087027f06: nid: 1 \ > > unused scan count 0 new scan count 0 total_scan 0 \ > > last shrinker return val 0 > > > > <...>-619914 [005] .... 467456.345380: mm_shrink_slab_end: \ > > super_cache_scan+0x0/0x1a0 0000000087027f06: nid: 1 unused \ > > scan count 0 new scan count 0 total_scan 0 \ > > last shrinker return val 0 > > > > This lead to unnecessary checks and atomic operations, which can be > > avoided by checking next_deferred for not being zero before calling > > add_nr_deferred(). In this case the mm_shrink_slab_end trace point > > will get a potentially slightly outdated "new scan count" value, but > > it's totally fine. > > Sufficient improvement to justify added complexity for anybody reading > that code? I don't have any numbers and really doubt the difference is significant, however the added complexity is also small: one "if" statement. Anyway, if you feel strongly against this change, I'm fine with dropping it. > > > > > Signed-off-by: Roman Gushchin > > --- > > mm/vmscan.c | 5 ++++- > > 1 file changed, 4 insertions(+), 1 deletion(-) > > > > diff --git a/mm/vmscan.c b/mm/vmscan.c > > index d4a7d2bd276d..19d3d4fa1aad 100644 > > --- a/mm/vmscan.c > > +++ b/mm/vmscan.c > > @@ -808,7 +808,10 @@ static unsigned long do_shrink_slab(struct shrink_control *shrinkctl, > > * move the unused scan count back into the shrinker in a > > * manner that handles concurrent updates. > > */ > > - new_nr = add_nr_deferred(next_deferred, shrinker, shrinkctl); > > + if (next_deferred) > > + new_nr = add_nr_deferred(next_deferred, shrinker, shrinkctl); > > + else > > + new_nr = nr; > > > > trace_mm_shrink_slab_end(shrinker, shrinkctl->nid, freed, nr, new_nr, total_scan); > > return freed; > > And if we still want to do this optimization, why not put it into > add_nr_deferred()? Because of the semantics of add_nr_deferred(), which returns the deferred value. It's not used for anything except tracing, so maybe it's a place for another change. Thanks!