From mboxrd@z Thu Jan 1 00:00:00 1970 From: Mike Snitzer Subject: Re: [patch 0/5] device mapper percpu patches Date: Wed, 7 Nov 2018 17:29:05 -0500 Message-ID: <20181107222904.GB24491@redhat.com> References: <20181106213458.308968760@debian-a64.vm> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: Content-Disposition: inline In-Reply-To: <20181106213458.308968760@debian-a64.vm> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: dm-devel-bounces@redhat.com Errors-To: dm-devel-bounces@redhat.com To: Mikulas Patocka Cc: axboe@kernel.dk, dm-devel@redhat.com, "Alasdair G. Kergon" List-Id: dm-devel.ids On Tue, Nov 06 2018 at 4:34pm -0500, Mikulas Patocka wrote: > Hi > > These are the device mapper percpu patches. > > Note that I didn't test request-based device mapper because I don't have > hardware for it (the patches don't convert request-base targets to percpu > values, but there are a few inevitable changes in dm-rq.c). Patches 1 - 3 make sense. But the use of percpu inflight counters isn't something I can get upstream. Any more scalable counter still needs to be wired up to the block stats interfaces (the one you did in patch 5 is only for the "inflight" fsffs file, there is also the generic diskstats callout to part_in_flight(), etc). Wiring up both part_in_flight() and part_in_flight_rw() to optionally callout to a new callback isn't going to fly.. especially if that callout is looping up the sum of percpu counters. I checked with Jens and now that in 4.21 all of the old request-based IO path is gone (and given that blk-mq bypasses use of ->in_flight[]): the only consumer of the existing ->in_flight[] is the bio-based IO path. Given that now only bio-based is consuming it, and your work was focused on making bio-based DM's "pending" IO accounting more scalable, it is best to just change block core's ->in_flight[] directly. But Jens is against switching to using percpu counters because they are really slow when summing the counts. And diskstats does that frequently. Jens said at least 2 other attempts were made and rejected to switch over to percpu counters. Jens' suggestion is to implement a new generic rolling per-node counter. Would you be open to trying that? Mike