From: Reinette Chatre <reinette.chatre@intel.com>
To: Peter Newman <peternewman@google.com>, <fenghua.yu@intel.com>
Cc: <bp@alien8.de>, <derkling@google.com>, <eranian@google.com>,
<hpa@zytor.com>, <james.morse@arm.com>, <jannh@google.com>,
<kpsingh@google.com>, <linux-kernel@vger.kernel.org>,
<mingo@redhat.com>, <tglx@linutronix.de>, <x86@kernel.org>
Subject: Re: [PATCH v5 1/1] x86/resctrl: Fix task CLOSID/RMID update race
Date: Thu, 15 Dec 2022 15:51:55 -0800 [thread overview]
Message-ID: <bbc57154-b7f5-4b9a-65ba-ca8dc0fe0dfe@intel.com> (raw)
In-Reply-To: <20221214114447.1935755-2-peternewman@google.com>
Hi Peter,
On 12/14/2022 3:44 AM, Peter Newman wrote:
> When the user moves a running task to a new rdtgroup using the tasks
> file interface or by deleting its rdtgroup, the resulting change in
> CLOSID/RMID must be immediately propagated to the PQR_ASSOC MSR on the
> task(s) CPUs.
>
> x86 allows reordering loads with prior stores, so if the task starts
> running between a task_curr() check that the CPU hoisted before the
> stores in the CLOSID/RMID update then it can start running with the old
> CLOSID/RMID until it is switched again because __rdtgroup_move_task()
> failed to determine that it needs to be interrupted to obtain the new
> CLOSID/RMID.
>
> Refer to the diagram below:
>
> CPU 0 CPU 1
> ----- -----
> __rdtgroup_move_task():
> curr <- t1->cpu->rq->curr
> __schedule():
> rq->curr <- t1
> resctrl_sched_in():
> t1->{closid,rmid} -> {1,1}
> t1->{closid,rmid} <- {2,2}
> if (curr == t1) // false
> IPI(t1->cpu)
>
> A similar race impacts rdt_move_group_tasks(), which updates tasks in a
> deleted rdtgroup.
>
> In both cases, use smp_mb() to order the task_struct::{closid,rmid}
> stores before the loads in task_curr(). In particular, in the
> rdt_move_group_tasks() case, simply execute an smp_mb() on every
> iteration with a matching task.
>
> It is possible to use a single smp_mb() in rdt_move_group_tasks(), but
> this would require two passes and a means of remembering which
> task_structs were updated in the first loop. However, benchmarking
> results below showed too little performance impact in the simple
> approach to justify implementing the two-pass approach.
>
> Times below were collected using `perf stat` to measure the time to
> remove a group containing a 1600-task, parallel workload.
>
> CPU: Intel(R) Xeon(R) Platinum P-8136 CPU @ 2.00GHz (112 threads)
>
> # mkdir /sys/fs/resctrl/test
> # echo $$ > /sys/fs/resctrl/test/tasks
> # perf bench sched messaging -g 40 -l 100000
>
> task-clock time ranges collected using:
>
> # perf stat rmdir /sys/fs/resctrl/test
>
> Baseline: 1.54 - 1.60 ms
> smp_mb() every matching task: 1.57 - 1.67 ms
>
For a fix a Fixes: tag is expected. It looks like the following
may be relevant:
Fixes: ae28d1aae48a ("x86/resctrl: Use an IPI instead of task_work_add() to update PQR_ASSOC MSR")
Fixes: 0efc89be9471 ("x86/intel_rdt: Update task closid immediately on CPU in rmdir and unmount")
> Signed-off-by: Peter Newman <peternewman@google.com>
Also, please do let the stable team know about this via:
Cc: stable@vger.kernel.org
> ---
There is no need to submit with a cover letter, but please do keep the history with this patch
by including it here below the "---".
> arch/x86/kernel/cpu/resctrl/rdtgroup.c | 12 +++++++++++-
> 1 file changed, 11 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/kernel/cpu/resctrl/rdtgroup.c b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> index e5a48f05e787..5993da21d822 100644
> --- a/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> +++ b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> @@ -580,8 +580,10 @@ static int __rdtgroup_move_task(struct task_struct *tsk,
> /*
> * Ensure the task's closid and rmid are written before determining if
> * the task is current that will decide if it will be interrupted.
> + * This pairs with the full barrier between the rq->curr update and
> + * resctrl_sched_in() during context switch.
> */
> - barrier();
> + smp_mb();
>
> /*
> * By now, the task's closid and rmid are set. If the task is current
> @@ -2401,6 +2403,14 @@ static void rdt_move_group_tasks(struct rdtgroup *from, struct rdtgroup *to,
> WRITE_ONCE(t->closid, to->closid);
> WRITE_ONCE(t->rmid, to->mon.rmid);
>
> + /*
> + * Order the closid/rmid stores above before the loads
> + * in task_curr(). This pairs with the full barrier
> + * between the rq->curr update and resctrl_sched_in()
> + * during context switch.
> + */
> + smp_mb();
> +
> /*
> * If the task is on a CPU, set the CPU in the mask.
> * The detection is inaccurate as tasks might move or
Thank you very much for sticking with this and always paying attention
to the details along the way.
Reviewed-by: Reinette Chatre <reinette.chatre@intel.com>
Reinette
next prev parent reply other threads:[~2022-12-15 23:52 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-12-14 11:44 [PATCH v5 0/1] Subject: x86/resctrl: Fix task CLOSID update race Peter Newman
2022-12-14 11:44 ` [PATCH v5 1/1] x86/resctrl: Fix task CLOSID/RMID " Peter Newman
2022-12-15 23:51 ` Reinette Chatre [this message]
2022-12-16 10:26 ` Peter Newman
2022-12-16 19:36 ` Reinette Chatre
2022-12-19 10:22 ` Peter Newman
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=bbc57154-b7f5-4b9a-65ba-ca8dc0fe0dfe@intel.com \
--to=reinette.chatre@intel.com \
--cc=bp@alien8.de \
--cc=derkling@google.com \
--cc=eranian@google.com \
--cc=fenghua.yu@intel.com \
--cc=hpa@zytor.com \
--cc=james.morse@arm.com \
--cc=jannh@google.com \
--cc=kpsingh@google.com \
--cc=linux-kernel@vger.kernel.org \
--cc=mingo@redhat.com \
--cc=peternewman@google.com \
--cc=tglx@linutronix.de \
--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).