From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-qk0-f198.google.com (mail-qk0-f198.google.com [209.85.220.198]) by kanga.kvack.org (Postfix) with ESMTP id 2014E6B0006 for ; Fri, 22 Jun 2018 08:44:58 -0400 (EDT) Received: by mail-qk0-f198.google.com with SMTP id c139-v6so5460935qkg.6 for ; Fri, 22 Jun 2018 05:44:58 -0700 (PDT) Received: from mx1.redhat.com (mx3-rdu2.redhat.com. [66.187.233.73]) by mx.google.com with ESMTPS id l3-v6si5205140qte.81.2018.06.22.05.44.56 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Fri, 22 Jun 2018 05:44:56 -0700 (PDT) Date: Fri, 22 Jun 2018 08:44:52 -0400 (EDT) From: Mikulas Patocka Subject: Re: dm bufio: Reduce dm_bufio_lock contention In-Reply-To: <20180622090151.GS10465@dhcp22.suse.cz> Message-ID: References: <20180614073153.GB9371@dhcp22.suse.cz> <20180615073201.GB24039@dhcp22.suse.cz> <20180615115547.GH24039@dhcp22.suse.cz> <20180615130925.GI24039@dhcp22.suse.cz> <20180619104312.GD13685@dhcp22.suse.cz> <20180622090151.GS10465@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: owner-linux-mm@kvack.org List-ID: To: Michal Hocko Cc: jing xia , Mike Snitzer , agk@redhat.com, dm-devel@redhat.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org On Fri, 22 Jun 2018, Michal Hocko wrote: > On Thu 21-06-18 21:17:24, Mikulas Patocka wrote: > [...] > > > But seriously, isn't the best way around the throttling issue to use > > > PF_LESS_THROTTLE? > > > > Yes - it could be done by setting PF_LESS_THROTTLE. But I think it would > > be better to change it just in one place than to add PF_LESS_THROTTLE to > > every block device driver (because adding it to every block driver results > > in more code). > > Why would every block device need this? I thought we were talking about > mempool allocator and the md variant of it. They are explicitly doing > their own back off so PF_LESS_THROTTLE sounds appropriate to me. Because every block driver is suspicible to this problem. Two years ago, there was a bug that when the user was swapping to dm-crypt device, memory management would stall the allocations done by dm-crypt by 100ms - that slowed down swapping rate and made the machine unuseable. Then, people are complaining about the same problem in dm-bufio. Next time, it will be something else. (you will answer : "we will not fix bugs unless people are reporting them" :-) > > What about this patch? If __GFP_NORETRY and __GFP_FS is not set (i.e. the > > request comes from a block device driver or a filesystem), we should not > > sleep. > > Why? How are you going to audit all the callers that the behavior makes > sense and moreover how are you going to ensure that future usage will > still make sense. The more subtle side effects gfp flags have the harder > they are to maintain. I did audit them - see the previous email in this thread: https://www.redhat.com/archives/dm-devel/2018-June/thread.html Mikulas > > Signed-off-by: Mikulas Patocka > > Cc: stable@vger.kernel.org > > > > Index: linux-2.6/mm/vmscan.c > > =================================================================== > > --- linux-2.6.orig/mm/vmscan.c > > +++ linux-2.6/mm/vmscan.c > > @@ -2674,6 +2674,7 @@ static bool shrink_node(pg_data_t *pgdat > > * the LRU too quickly. > > */ > > if (!sc->hibernation_mode && !current_is_kswapd() && > > + (sc->gfp_mask & (__GFP_NORETRY | __GFP_FS)) != __GFP_NORETRY && > > current_may_throttle() && pgdat_memcg_congested(pgdat, root)) > > wait_iff_congested(BLK_RW_ASYNC, HZ/10); > > > > -- > Michal Hocko > SUSE Labs >