From mboxrd@z Thu Jan 1 00:00:00 1970 From: Kent Overstreet Subject: Re: bcache_writebac task 100% CPU Date: Mon, 19 May 2014 09:55:50 -0700 Message-ID: <20140519165550.GD2276@kmo> References: <53786E02.8060300@rolffokkens.nl> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Received: from mail-pb0-f46.google.com ([209.85.160.46]:32773 "EHLO mail-pb0-f46.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932574AbaESQzx (ORCPT ); Mon, 19 May 2014 12:55:53 -0400 Received: by mail-pb0-f46.google.com with SMTP id rq2so6120106pbb.19 for ; Mon, 19 May 2014 09:55:53 -0700 (PDT) Content-Disposition: inline In-Reply-To: <53786E02.8060300@rolffokkens.nl> Sender: linux-bcache-owner@vger.kernel.org List-Id: linux-bcache@vger.kernel.org To: Rolf Fokkens Cc: linux-bcache@vger.kernel.org On Sun, May 18, 2014 at 10:23:30AM +0200, Rolf Fokkens wrote: > Sometimes bcache_writebac task consumes 100% CPU during normal operation, but after a while (like a minute) this drops to normal. > > It can be triggered in a reproducable fashion on my system by: > > echo 0 > /sys/block/bcache0/bcache/writeback_percent > > After this bcache_writebac starts to burn CPU cycles, and it never seems to stop doing so. changing the writeback_percent value back to 25 doesn't change anything. > > Apart from a busy CPU there appear to be no problem (e.g. no "task bcache_writebac:286 blocked for more than 120 seconds" messages). ...Changing it back to 25% doesn't do anything? _that_ is odd... Can you send me the output of writeback_rate_debug? Check it once before you've done anything, when things are working normally (after a fresh boot when writeback_percent is nonzero, probably), then grab it again after you've set writeback_percent to 0 and then back to 25, when it's still spinning. There seems to be two separate bugs here, the one I'm particularly concerned about is why setting writeback_percent back to 25 doesn't restore the old behaviour. Also, what kernel are you running?