From: "Chris Mason" <clm@fb.com>
To: Vincent Guittot <vincent.guittot@linaro.org>
Cc: Rik van Riel <riel@surriel.com>,
Peter Zijlstra <peterz@infradead.org>,
Johannes Weiner <hannes@cmpxchg.org>,
linux-kernel <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] fix scheduler regression from "sched/fair: Rework load_balance()"
Date: Mon, 26 Oct 2020 12:48:30 -0400 [thread overview]
Message-ID: <B23038A4-945F-48E9-8D38-EABE8204F208@fb.com> (raw)
In-Reply-To: <20201026162029.GA11367@vingu-book>
On 26 Oct 2020, at 12:20, Vincent Guittot wrote:
> Le lundi 26 oct. 2020 à 12:04:45 (-0400), Rik van Riel a écrit :
>> On Mon, 26 Oct 2020 16:42:14 +0100
>> Vincent Guittot <vincent.guittot@linaro.org> wrote:
>>> On Mon, 26 Oct 2020 at 16:04, Rik van Riel <riel@surriel.com> wrote:
>>
>>>> Could utilization estimates be off, either lagging or
>>>> simply having a wrong estimate for a task, resulting
>>>> in no task getting pulled sometimes, while doing a
>>>> migrate_task imbalance always moves over something?
>>>
>>> task and cpu utilization are not always up to fully synced and may
>>> lag
>>> a bit which explains that sometimes LB can fail to migrate for a
>>> small
>>> diff
>>
>> OK, running with this little snippet below, I see latencies
>> improve back to near where they used to be:
>>
>> Latency percentiles (usec) runtime 150 (s)
>> 50.0th: 13
>> 75.0th: 31
>> 90.0th: 69
>> 95.0th: 90
>> *99.0th: 761
>> 99.5th: 2268
>> 99.9th: 9104
>> min=1, max=16158
>>
>> I suspect the right/cleaner approach might be to use
>> migrate_task more in !CPU_NOT_IDLE cases?
>>
>> Running a task to an idle CPU immediately, instead of refusing
>> to have the load balancer move it, improves latencies for fairly
>> obvious reasons.
>>
>> I am not entirely clear on why the load balancer should need to
>> be any more conservative about moving tasks than the wakeup
>> path is in eg. select_idle_sibling.
>
>
> what you are suggesting is something like:
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 4978964e75e5..3b6fbf33abc2 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -9156,7 +9156,8 @@ static inline void calculate_imbalance(struct
> lb_env *env, struct sd_lb_stats *s
> * emptying busiest.
> */
> if (local->group_type == group_has_spare) {
> - if (busiest->group_type > group_fully_busy) {
> + if ((busiest->group_type > group_fully_busy) &&
> + !(env->sd->flags & SD_SHARE_PKG_RESOURCES)) {
> /*
> * If busiest is overloaded, try to fill spare
> * capacity. This might end up creating spare
> capacity
>
> which also fixes the problem for me and alignes LB with wakeup path
> regarding the migration
> in the LLC
Vincent’s patch on top of 5.10-rc1 looks pretty great:
Latency percentiles (usec) runtime 90 (s) (3320 total samples)
50.0th: 161 (1687 samples)
75.0th: 200 (817 samples)
90.0th: 228 (488 samples)
95.0th: 254 (164 samples)
*99.0th: 314 (131 samples)
99.5th: 330 (17 samples)
99.9th: 356 (13 samples)
min=29, max=358
Next we test in prod, which probably won’t have answers until
tomorrow. Thanks again Vincent!
-chris
next prev parent reply other threads:[~2020-10-26 16:50 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-10-23 23:49 [PATCH] fix scheduler regression from "sched/fair: Rework load_balance()" Chris Mason
2020-10-26 8:39 ` Vincent Guittot
2020-10-26 12:45 ` Chris Mason
2020-10-26 14:24 ` Vincent Guittot
2020-10-26 14:38 ` Rik van Riel
2020-10-26 14:56 ` Vincent Guittot
2020-10-26 15:04 ` Rik van Riel
2020-10-26 15:42 ` Vincent Guittot
2020-10-26 15:54 ` Vincent Guittot
2020-10-26 16:04 ` Rik van Riel
2020-10-26 16:20 ` Vincent Guittot
2020-10-26 16:48 ` Chris Mason [this message]
2020-10-26 16:52 ` Vincent Guittot
2020-10-30 2:10 ` Rik van Riel
2020-10-30 9:16 ` Vincent Guittot
2020-10-26 15:05 ` Chris Mason
2020-10-26 15:18 ` Vincent Guittot
2020-10-26 15:28 ` Chris Mason
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=B23038A4-945F-48E9-8D38-EABE8204F208@fb.com \
--to=clm@fb.com \
--cc=hannes@cmpxchg.org \
--cc=linux-kernel@vger.kernel.org \
--cc=peterz@infradead.org \
--cc=riel@surriel.com \
--cc=vincent.guittot@linaro.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).