From: Reinette Chatre <reinette.chatre@intel.com>
To: Valentin Schneider <valentin.schneider@arm.com>
Cc: tglx@linutronix.de, fenghua.yu@intel.com, bp@alien8.de,
tony.luck@intel.com, kuo-lang.tseng@intel.com,
shakeelb@google.com, mingo@redhat.com, babu.moger@amd.com,
james.morse@arm.com, hpa@zytor.com, x86@kernel.org,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH 0/3] x86/resctrl: Fix a few issues in moving a task to a resource group
Date: Mon, 14 Dec 2020 10:38:58 -0800 [thread overview]
Message-ID: <07ccc96d-a875-af0e-5169-24b1f84c46da@intel.com> (raw)
In-Reply-To: <jhjk0tot6jg.mognet@arm.com>
Hi Valentin,
On 12/11/2020 12:46 PM, Valentin Schneider wrote:
> On 03/12/20 23:25, Reinette Chatre wrote:
>> Valentin's series in [2] ends by adding memory barriers to support the
>> updating of the task_struct from one CPU and the usage of the task_struct data
>> from another CPU. This work is still needed and as discussed with Valentin in
>> that thread the work would be re-evaluated by him after seeing how this series
>> turns out.
>>
Thank you very much for taking a look.
>
> So the "problematic" pattern is still there: a context switch can happen
> concurrently with a write to the switching-to-tasks's {closid, rmid}.
> Accesses to these fields would thus need to be wrapped by READ_ONCE() &
> WRITE_ONCE().
ok.
>
> Thinking a bit more (too much?) about it, we could limit ourselves to
> wrapping only reads not protected by the rdtgroup_mutex: the only two
> task_struct {closid, rmid} writers are
> - rdtgroup_move_task()
> - rdt_move_group_tasks()
> and they are both invoked while holding said mutex. Thus, a reader holding
> the mutex cannot race with a write, so load tearing ought to be safe.
The reads that are not protected by the rdtgroup_mutex can be found in
__resctrl_sched_in(). It thus sounds to me like your proposed changes to
this function found in your patch [1] is what is needed? It is not clear
to me how the pairing would work in this case though. If I understand
correctly the goal is for the write to the closid/rmid in the functions
you mention above to be paired with the reads in resctrl_sched_in() and
it is not clear how adding a single READ_ONCE would accomplish this
pairing by itself.
It is also not entirely clear to me what the problematic scenario could
be. If I understand correctly, the risk is (as you explained in your
commit message), that a CPU could have its {closid, rmid} fields read
locally (resctrl_sched_in()) while they are concurrently being written
to from another CPU (in rdtgroup_move_task() and rdt_move_group_tasks()
as you state above). If this happens then a task being moved may be
scheduled in with its old closid/rmid. The update of closid/rmid in
rdtgroup_move_task()/rdt_move_group_tasks() is followed by
smp_call_function_xx() where the registers are updated with preemption
disabled and thus protected against __switch_to. If a task was thus
incorrectly scheduled in with old closid/rmid, would it not be corrected
at this point?
Thank you
Reinette
[1]
https://lore.kernel.org/lkml/20201123022433.17905-4-valentin.schneider@arm.com/
next prev parent reply other threads:[~2020-12-14 18:40 UTC|newest]
Thread overview: 22+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-12-03 23:25 [PATCH 0/3] x86/resctrl: Fix a few issues in moving a task to a resource group Reinette Chatre
2020-12-03 23:25 ` [PATCH 1/3] x86/resctrl: Move setting task's active CPU in a mask into helpers Reinette Chatre
2020-12-07 18:29 ` Borislav Petkov
2020-12-07 21:24 ` Reinette Chatre
2020-12-08 9:49 ` Borislav Petkov
2020-12-08 16:35 ` Reinette Chatre
2020-12-09 16:47 ` James Morse
2020-12-10 0:21 ` Reinette Chatre
2020-12-03 23:25 ` [PATCH 2/3] x86/resctrl: Update PQR_ASSOC MSR synchronously when moving task to resource group Reinette Chatre
2020-12-09 16:51 ` James Morse
2020-12-10 0:22 ` Reinette Chatre
2020-12-11 20:46 ` Valentin Schneider
2020-12-14 18:41 ` Reinette Chatre
2020-12-16 17:41 ` Valentin Schneider
2020-12-16 18:26 ` Reinette Chatre
2020-12-17 10:39 ` Valentin Schneider
2020-12-03 23:25 ` [PATCH 3/3] x86/resctrl: Don't move a task to the same " Reinette Chatre
2020-12-11 20:46 ` [PATCH 0/3] x86/resctrl: Fix a few issues in moving a task to a " Valentin Schneider
2020-12-14 18:38 ` Reinette Chatre [this message]
2020-12-16 17:41 ` Valentin Schneider
2020-12-16 18:26 ` Reinette Chatre
2020-12-17 12:19 ` [PATCH 4/3] x86/intel_rdt: Apply READ_ONCE/WRITE_ONCE to task_struct .rmid & .closid Valentin Schneider
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=07ccc96d-a875-af0e-5169-24b1f84c46da@intel.com \
--to=reinette.chatre@intel.com \
--cc=babu.moger@amd.com \
--cc=bp@alien8.de \
--cc=fenghua.yu@intel.com \
--cc=hpa@zytor.com \
--cc=james.morse@arm.com \
--cc=kuo-lang.tseng@intel.com \
--cc=linux-kernel@vger.kernel.org \
--cc=mingo@redhat.com \
--cc=shakeelb@google.com \
--cc=tglx@linutronix.de \
--cc=tony.luck@intel.com \
--cc=valentin.schneider@arm.com \
--cc=x86@kernel.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).