From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Return-Path: From: Paolo Subject: Re: [PATCH RFC 10/22] block, bfq: add full hierarchical scheduling and cgroups support To: Tejun Heo References: <1454364778-25179-1-git-send-email-paolo.valente@linaro.org> <1454364778-25179-11-git-send-email-paolo.valente@linaro.org> <20160211222824.GD3741@mtj.duckdns.org> Cc: Jens Axboe , Fabio Checconi , Arianna Avanzini , linux-block@vger.kernel.org, linux-kernel@vger.kernel.org, ulf.hansson@linaro.org, linus.walleij@linaro.org, broonie@kernel.org Message-ID: <57174CA7.5000706@linaro.org> Date: Wed, 20 Apr 2016 11:32:23 +0200 MIME-Version: 1.0 In-Reply-To: <20160211222824.GD3741@mtj.duckdns.org> Content-Type: text/plain; charset=windows-1252; format=flowed List-ID: [Resending in plain text] Il 11/02/2016 23:28, Tejun Heo ha scritto: > Hello, > > On Mon, Feb 01, 2016 at 11:12:46PM +0100, Paolo Valente wrote: >> From: Arianna Avanzini >> >> Complete support for full hierarchical scheduling, with a cgroups >> interface. The name of the added policy is bfq. >> >> Weights can be assigned explicitly to groups and processes through the >> cgroups interface, differently from what happens, for single >> processes, if the cgroups interface is not used (as explained in the >> description of the previous patch). In particular, since each node has >> a full scheduler, each group can be assigned its own weight. > > * It'd be great if how cgroup support is achieved is better > documented. > > * How's writeback handled? > > * After all patches are applied, both CONFIG_BFQ_GROUP_IOSCHED and > CONFIG_CFQ_GROUP_IOSCHED exist. > > * The default weight and weight range don't seem to follow the defined > interface on the v2 hierarchy. The default value should be 100. > > * With all patches applied, booting triggers a RCU context warning. > Please build with lockdep and RCU debugging turned on and fix the > issue. > > * I was testing on the v2 hierarchy with two top-level cgroups one > hosting sequential workload and the other completely random. While > they eventually converged to a reasonable state, starting up the > sequential workload while the random workload was running was > extremely slow. It crawled for quite a while. This malfunction seems related to a blkcg behavior that I did not expect: the sequential writer changes group continuously. It moves from the root group to its correct group, and back. Here is the output of egrep 'insert_request|changed cgroup' trace over a trace taken with the original version of cfq (seq_write is of course the group of the writer): kworker/u8:2-96 [000] d... 204.561086: 8,0 m N cfq96A /seq_write changed cgroup kworker/u8:2-96 [000] d... 204.561097: 8,0 m N cfq96A / changed cgroup kworker/u8:2-96 [000] d... 204.561353: 8,0 m N cfq96A / insert_request kworker/u8:2-96 [000] d... 204.561369: 8,0 m N cfq96A /seq_write insert_request kworker/u8:2-96 [000] d... 204.561379: 8,0 m N cfq96A /seq_write insert_request kworker/u8:2-96 [000] d... 204.566509: 8,0 m N cfq96A /seq_write changed cgroup kworker/u8:2-96 [000] d... 204.566517: 8,0 m N cfq96A / changed cgroup kworker/u8:2-96 [000] d... 204.566690: 8,0 m N cfq96A / insert_request kworker/u8:2-96 [000] d... 204.567203: 8,0 m N cfq96A /seq_write insert_request kworker/u8:2-96 [000] d... 204.567216: 8,0 m N cfq96A /seq_write insert_request kworker/u8:2-96 [000] d... 204.567328: 8,0 m N cfq96A /seq_write insert_request kworker/u8:2-96 [000] d... 204.571622: 8,0 m N cfq96A /seq_write changed cgroup kworker/u8:2-96 [000] d... 204.571640: 8,0 m N cfq96A / changed cgroup kworker/u8:2-96 [000] d... 204.572021: 8,0 m N cfq96A / insert_request kworker/u8:2-96 [000] d... 204.572463: 8,0 m N cfq96A /seq_write insert_request ... For reasons that I don't yet know, group changes are much more frequent with bfq, which ultimately causes bfq to fail to isolate the writer from the reader. While I go on trying to understand why, could you please tell me whether this fluctuation is normal, and/or point me to documentation from which I can better understand this behavior, without bothering you further? Thanks, Paolo > > * And "echo 100 > io.weight" hung the writing process. > > Thanks. >