From mboxrd@z Thu Jan 1 00:00:00 1970 Subject: Re: [PATCH 04/10] lib: percpu_counter_mod64 From: Peter Zijlstra In-Reply-To: <20070421122139.f5259c82.akpm@linux-foundation.org> References: <20070420155154.898600123@chello.nl> <20070420155502.787144532@chello.nl> <20070421025517.d9f9bc14.akpm@linux-foundation.org> <1177153346.2934.36.camel@lappy> <20070421122139.f5259c82.akpm@linux-foundation.org> Content-Type: text/plain Date: Sat, 21 Apr 2007 21:30:25 +0200 Message-Id: <1177183825.7316.25.camel@twins> Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org Return-Path: To: Andrew Morton Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, miklos@szeredi.hu, neilb@suse.de, dgc@sgi.com, tomoki.sekiyama.qu@hitachi.com, nikita@clusterfs.com, trond.myklebust@fys.uio.no, yingchao.zhou@gmail.com List-ID: On Sat, 2007-04-21 at 12:21 -0700, Andrew Morton wrote: > On Sat, 21 Apr 2007 13:02:26 +0200 Peter Zijlstra wrote: > > > > > + cpu = get_cpu(); > > > > + pcount = per_cpu_ptr(fbc->counters, cpu); > > > > + count = *pcount + amount; > > > > + if (count >= FBC_BATCH || count <= -FBC_BATCH) { > > > > + spin_lock(&fbc->lock); > > > > + fbc->count += count; > > > > + *pcount = 0; > > > > + spin_unlock(&fbc->lock); > > > > + } else { > > > > + *pcount = count; > > > > + } > > > > + put_cpu(); > > > > +} > > > > +EXPORT_SYMBOL(percpu_counter_mod64); > > > > > > Bloaty. Surely we won't be needing this on 32-bit kernels? Even monster > > > PAE has only 64,000,000 pages and won't be using deltas of more than 4 > > > gigapages? > > > > > > > > another changelog bug> > > > > Yeah, /me chastises himself for that... > > > > This is because percpu_counter is s64 instead of the native long; I need > > to halve the counter at some point (bdi_writeout_norm) and do that by > > subtracting half the current value. > > ah, the mysterious bdi_writeout_norm(). > > I don't think it's possible to precisely halve a percpu_counter - there has > to be some error involved. I guess that's acceptable within the > inscrutable bdi_writeout_norm(). > > otoh, there's a chance that the attempt to halve the counter will take the > counter negative, due to races. Does the elusive bdi_writeout_norm() > handle that? If not, it should. If it does, then there should be comments > around the places where this is being handled, because it is subtle, and unobvious, > and others might break it by accident. The counter it is halving is only ever incremented, so we might be off a little, but only to the safe side. I shall do the comment thing along with all the other missing comments :-) -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org