From: Joel Fernandes <joel@joelfernandes.org>
To: peterz@infradead.org
Cc: Vineeth Pillai <viremana@linux.microsoft.com>,
Julien Desfossez <jdesfossez@digitalocean.com>,
Tim Chen <tim.c.chen@linux.intel.com>,
Aaron Lu <aaron.lwe@gmail.com>,
Aubrey Li <aubrey.intel@gmail.com>,
Dhaval Giani <dhaval.giani@oracle.com>,
Chris Hyser <chris.hyser@oracle.com>,
Nishanth Aravamudan <naravamudan@digitalocean.com>,
mingo@kernel.org, tglx@linutronix.de, pjt@google.com,
torvalds@linux-foundation.org, linux-kernel@vger.kernel.org,
fweisbec@gmail.com, keescook@chromium.org, kerrnel@google.com,
Phil Auld <pauld@redhat.com>,
Valentin Schneider <valentin.schneider@arm.com>,
Mel Gorman <mgorman@techsingularity.net>,
Pawan Gupta <pawan.kumar.gupta@linux.intel.com>,
Paolo Bonzini <pbonzini@redhat.com>,
vineeth@bitbyteword.org, Chen Yu <yu.c.chen@intel.com>,
Christian Brauner <christian.brauner@ubuntu.com>,
Agata Gruza <agata.gruza@intel.com>,
Antonio Gomez Iglesias <antonio.gomez.iglesias@intel.com>,
graf@amazon.com, konrad.wilk@oracle.com, dfaggioli@suse.com,
rostedt@goodmis.org, derkling@google.com, benbjiang@tencent.com,
Vineeth Remanan Pillai <vpillai@digitalocean.com>,
Aaron Lu <aaron.lu@linux.alibaba.com>
Subject: Re: [RFC PATCH v7 08/23] sched: Add core wide task selection and scheduling.
Date: Mon, 31 Aug 2020 10:24:27 -0400 [thread overview]
Message-ID: <20200831142427.GA3437943@google.com> (raw)
In-Reply-To: <20200829074719.GJ1362448@hirez.programming.kicks-ass.net>
Hi Peter,
On Sat, Aug 29, 2020 at 09:47:19AM +0200, peterz@infradead.org wrote:
> On Fri, Aug 28, 2020 at 06:02:25PM -0400, Vineeth Pillai wrote:
> > On 8/28/20 4:51 PM, Peter Zijlstra wrote:
>
> > > So where do things go side-ways?
>
> > During hotplug stress test, we have noticed that while a sibling is in
> > pick_next_task, another sibling can go offline or come online. What
> > we have observed is smt_mask get updated underneath us even if
> > we hold the lock. From reading the code, looks like we don't hold the
> > rq lock when the mask is updated. This extra logic was to take care of that.
>
> Sure, the mask is updated async, but _where_ is the actual problem with
> that?
>
> On Fri, Aug 28, 2020 at 06:23:55PM -0400, Joel Fernandes wrote:
> > Thanks Vineeth. Peter, also the "v6+" series (which were some addons on v6)
> > detail the individual hotplug changes squashed into this patch:
> > https://lore.kernel.org/lkml/20200815031908.1015049-9-joel@joelfernandes.org/
> > https://lore.kernel.org/lkml/20200815031908.1015049-11-joel@joelfernandes.org/
>
> That one looks fishy, the pick is core wide, making that pick_seq per rq
> just doesn't make sense.
I think Vineeth was trying to handle the case where rq->core_pick happened to
be NULL for an offline CPU, and then schedule() is called when it came online
but its sched_seq != core-wide pick_seq. The reason for this situation is
because a sibling did selection for the offline CPU and ended up leaving its
rq->core_pick as NULL as the then-offline CPU was missing from the
cpu_smt_mask, but it incremented the core-wide pick_seq anyway.
Due to this, the pick_next_task() can crash after entering this if() block:
+ if (rq->core_pick_seq == rq->core->core_task_seq &&
+ rq->core_pick_seq != rq->core_sched_seq) {
How would you suggest to fix it? Maybe we can just assign rq->core_sched_seq
== rq->core_pick_seq for an offline CPU (or any CPU where rq->core_pick ==
NULL), so it does not end up using rq->core_pick and does a full core-wide
selcetion again when it comes online?
Or easier, check for rq->core_pick == NULL and skip this fast-path if() block
completely.
> > https://lore.kernel.org/lkml/20200815031908.1015049-12-joel@joelfernandes.org/
>
> This one reads like tinkering, there is no description of the actual
> problem just some code that makes a symptom go away.
>
> Sure, on hotplug the smt mask can change, but only for a CPU that isn't
> actually scheduling, so who cares.
>
> /me re-reads the hotplug code...
>
> ..ooOO is the problem that we clear the cpumasks on take_cpu_down()
> instead of play_dead() ?! That should be fixable.
I think Vineeth explained this in his email, there is logic across the loops
in the pick_next_task() that depend on the cpu_smt_mask not change. I am not
sure if play_dead() will fix it, the issue is seen in the code doing the
selection and the cpu_smt_mask changing under it due to possibly other CPUs
going offline.
Example, you have a splat and null pointer dereference possibilities in the
below loop if rq_i ->core_pick == NULL, because a sibling CPU came online but
a task was not selected for it in the for loops prior to this for loop:
/*
* Reschedule siblings
*
* NOTE: L1TF -- at this point we're no longer running the old task and
* sending an IPI (below) ensures the sibling will no longer be running
* their task. This ensures there is no inter-sibling overlap between
* non-matching user state.
*/
for_each_cpu(i, smt_mask) {
struct rq *rq_i = cpu_rq(i);
WARN_ON_ONCE(!rq_i->core_pick);
if (is_idle_task(rq_i->core_pick) && rq_i->nr_running)
rq_i->core_forceidle = true;
rq_i->core_pick->core_occupation = occ;
Probably the code can be rearchitected to not depend on cpu_smt_mask
changing. What I did in my old tree is I made a copy of the cpu_smt_mask in
the beginning of this function, and that makes all the problems go away. But
I was afraid of overhead of that copying.
(btw, I would not complain one bit if this function was nuked and rewritten
to be simpler).
> > https://lore.kernel.org/lkml/20200815031908.1015049-13-joel@joelfernandes.org/
>
> This is the only one that makes some sense, it makes rq->core consistent
> over hotplug.
Cool at least we got one thing right ;)
> > Agreed we can split the patches for the next series, however for final
> > upstream merge, I suggest we fix hotplug issues in this patch itself so that
> > we don't break bisectability.
>
> Meh, who sodding cares about hotplug :-). Also you can 'fix' such things
> by making sure you can't actually enable core-sched until after
> everything is in place.
Fair enough :)
thanks,
- Joel
next prev parent reply other threads:[~2020-08-31 14:24 UTC|newest]
Thread overview: 66+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-08-28 19:51 [RFC PATCH v7 00/23] Core scheduling v7 Julien Desfossez
2020-08-28 19:51 ` [RFC PATCH v7 01/23] sched: Wrap rq::lock access Julien Desfossez
2020-08-28 19:51 ` [RFC PATCH v7 02/23] sched: Introduce sched_class::pick_task() Julien Desfossez
2020-08-28 19:51 ` [RFC PATCH v7 03/23] sched: Core-wide rq->lock Julien Desfossez
2020-08-28 19:51 ` [RFC PATCH v7 04/23] sched/fair: Add a few assertions Julien Desfossez
2020-08-28 19:51 ` [RFC PATCH v7 05/23] sched: Basic tracking of matching tasks Julien Desfossez
2020-08-28 19:51 ` [RFC PATCH v7 06/23] bitops: Introduce find_next_or_bit Julien Desfossez
2020-09-03 5:13 ` Randy Dunlap
2020-08-28 19:51 ` [RFC PATCH v7 07/23] cpumask: Introduce a new iterator for_each_cpu_wrap_or Julien Desfossez
2020-08-28 19:51 ` [RFC PATCH v7 08/23] sched: Add core wide task selection and scheduling Julien Desfossez
2020-08-28 20:51 ` Peter Zijlstra
2020-08-28 22:02 ` Vineeth Pillai
2020-08-28 22:23 ` Joel Fernandes
2020-08-29 7:47 ` peterz
2020-08-31 13:01 ` Vineeth Pillai
2020-08-31 14:24 ` Joel Fernandes [this message]
2020-09-01 3:38 ` Joel Fernandes
2020-09-01 5:10 ` Joel Fernandes
2020-09-01 12:34 ` Vineeth Pillai
2020-09-01 17:30 ` Joel Fernandes
2020-09-01 21:23 ` Vineeth Pillai
2020-09-02 1:11 ` Joel Fernandes
2020-08-28 20:55 ` Peter Zijlstra
2020-08-28 22:15 ` Vineeth Pillai
2020-09-15 20:08 ` Joel Fernandes
2020-08-28 19:51 ` [RFC PATCH v7 09/23] sched/fair: Fix forced idle sibling starvation corner case Julien Desfossez
2020-08-28 21:25 ` Peter Zijlstra
2020-08-28 23:24 ` Vineeth Pillai
2020-08-28 19:51 ` [RFC PATCH v7 10/23] sched/fair: wrapper for cfs_rq->min_vruntime Julien Desfossez
2020-08-28 19:51 ` [RFC PATCH v7 11/23] sched/fair: core wide cfs task priority comparison Julien Desfossez
2020-08-28 21:29 ` Peter Zijlstra
2020-09-17 14:15 ` Vineeth Pillai
2020-09-17 20:39 ` Vineeth Pillai
2020-09-23 1:46 ` Joel Fernandes
2020-09-23 1:52 ` Joel Fernandes
2020-09-25 15:02 ` Joel Fernandes
2020-09-15 21:49 ` chris hyser
[not found] ` <81b208ad-b9e6-bfbf-631e-02e9f75d73a2@linux.intel.com>
2020-09-16 14:24 ` chris hyser
2020-09-16 20:53 ` chris hyser
2020-09-17 1:09 ` Li, Aubrey
2020-08-28 19:51 ` [RFC PATCH v7 12/23] sched: Trivial forced-newidle balancer Julien Desfossez
2020-09-02 7:08 ` Pavan Kondeti
2020-08-28 19:51 ` [RFC PATCH v7 13/23] sched: migration changes for core scheduling Julien Desfossez
2020-08-28 19:51 ` [RFC PATCH v7 14/23] irq_work: Add support to detect if work is pending Julien Desfossez
2020-08-28 19:51 ` [RFC PATCH v7 15/23] entry/idle: Add a common function for activites during idle entry/exit Julien Desfossez
2020-08-28 19:51 ` [RFC PATCH v7 16/23] arch/x86: Add a new TIF flag for untrusted tasks Julien Desfossez
2020-08-28 19:51 ` [RFC PATCH v7 17/23] kernel/entry: Add support for core-wide protection of kernel-mode Julien Desfossez
2020-09-01 15:54 ` Thomas Gleixner
2020-09-01 16:50 ` Joel Fernandes
2020-09-01 20:02 ` Thomas Gleixner
2020-09-02 1:29 ` Joel Fernandes
2020-09-02 7:53 ` Thomas Gleixner
2020-09-02 15:12 ` Joel Fernandes
2020-09-02 16:57 ` Dario Faggioli
2020-09-03 4:34 ` Joel Fernandes
2020-09-03 11:05 ` Vineeth Pillai
2020-09-03 13:20 ` Thomas Gleixner
2020-09-03 20:30 ` Joel Fernandes
2020-09-03 13:43 ` Dario Faggioli
2020-09-03 20:25 ` Joel Fernandes
2020-08-28 19:51 ` [RFC PATCH v7 18/23] entry/idle: Enter and exit kernel protection during idle entry and exit Julien Desfossez
2020-08-28 19:51 ` [RFC PATCH v7 19/23] entry/kvm: Protect the kernel when entering from guest Julien Desfossez
2020-08-28 19:51 ` [RFC PATCH v7 20/23] sched/coresched: config option for kernel protection Julien Desfossez
2020-08-28 19:51 ` [RFC PATCH v7 21/23] sched: cgroup tagging interface for core scheduling Julien Desfossez
2020-08-28 19:51 ` [RFC PATCH v7 22/23] Documentation: Add documentation on " Julien Desfossez
2020-08-28 19:51 ` [RFC PATCH v7 23/23] sched: Debug bits Julien Desfossez
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=20200831142427.GA3437943@google.com \
--to=joel@joelfernandes.org \
--cc=aaron.lu@linux.alibaba.com \
--cc=aaron.lwe@gmail.com \
--cc=agata.gruza@intel.com \
--cc=antonio.gomez.iglesias@intel.com \
--cc=aubrey.intel@gmail.com \
--cc=benbjiang@tencent.com \
--cc=chris.hyser@oracle.com \
--cc=christian.brauner@ubuntu.com \
--cc=derkling@google.com \
--cc=dfaggioli@suse.com \
--cc=dhaval.giani@oracle.com \
--cc=fweisbec@gmail.com \
--cc=graf@amazon.com \
--cc=jdesfossez@digitalocean.com \
--cc=keescook@chromium.org \
--cc=kerrnel@google.com \
--cc=konrad.wilk@oracle.com \
--cc=linux-kernel@vger.kernel.org \
--cc=mgorman@techsingularity.net \
--cc=mingo@kernel.org \
--cc=naravamudan@digitalocean.com \
--cc=pauld@redhat.com \
--cc=pawan.kumar.gupta@linux.intel.com \
--cc=pbonzini@redhat.com \
--cc=peterz@infradead.org \
--cc=pjt@google.com \
--cc=rostedt@goodmis.org \
--cc=tglx@linutronix.de \
--cc=tim.c.chen@linux.intel.com \
--cc=torvalds@linux-foundation.org \
--cc=valentin.schneider@arm.com \
--cc=vineeth@bitbyteword.org \
--cc=viremana@linux.microsoft.com \
--cc=vpillai@digitalocean.com \
--cc=yu.c.chen@intel.com \
/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).