Re: [PATCH v24 06/11] sched: Handle blocked-waiter migration (and return migration)

[K Prateek Nayak]

Hello John,

On 3/7/2026 12:18 PM, John Stultz wrote:
> On Mon, Dec 29, 2025 at 9:34 PM K Prateek Nayak <kprateek.nayak@xxxxxxx> wrote:
>> On 11/25/2025 4:00 AM, John Stultz wrote:
>>> +
>>> +     raw_spin_rq_lock(rq);
>>> +     rq_repin_lock(rq, rf);
>>> +     update_rq_clock(rq);
>>
>> I'm tempted to suggest extracting this pattern into a guard. We seem
>> to always do the zap + unpin + unlock somewhere in the middle and
>> lock + repin + update clock at the end of a function so how about:
>>
> [snipped guard logic]
>>
>> If the guard obfuscates the flow too much, I'm happy with the current
>> approach as well but having the guard + a comment on top of its usage is
>> lot more soothing on the eye IMHO :-)
>>
>> It also simplifies proxy_force_return() if my next set of comments aren't
>> too horrible. I'll leave it to you and Peter to decide which is best.
> 
> So first of all, apologies for being so slow to get to this! I very
> much apprecaite your feedback here, but the first two months of this
> year have been busy and I've not had the mental space to get my head
> back into the proxy-exec details. I definitely did not intend to
> neglect this for so long.
> 
> So on your suggestion, I'm torn. It does make the code simpler, but
> I've found some guard() usage to be less readable for me, as it can
> hide some subtleties.
> But I also felt this way when I first ran into scoped locking and I've
> started to warm to that.
> So maybe if it's ok, I'll leave this for now, but let's consider it
> for a future refactor?

Ack! Now that I look at it again, that guard does obscure the flow
a lot for a few lines saved in the diffstat (if at all!) so I too
think this can stay the same.

>>> +     /*
>>> +      * We drop the rq lock, and re-grab task_rq_lock to get
>>> +      * the pi_lock (needed for select_task_rq) as well.
>>> +      */
>>> +     this_rq = task_rq_lock(p, &this_rf);
>>
>> So I'm failing to see why we need to drop the rq_lock, re-grab it via
>> task_rq_lock() when we are eventually going to do a deactivate_task().
>>
>> Once we deactivate, wakeup path will stall at ttwu_runnable() since it
>> cannot grab the __task_rq_lock() with task_on_rq_migrating() so we
>> should be able to safely move the task around.
>>
>> Maybe my naive eyes are missing something (and perhaps there are some
>> details that arrive with sleeping owner stuff) but the following
>> boots fine and survives a sched-messaging and test-ww_mutex run
>> without any splat on my machine at this point:
> 
> Hum. The reason we need the pi_lock is for select_task_rq(), and since
> we hold the rq lock (which is under the pi_lock in the locking order),
> we need to drop it to take the pi_lock and rq_lock via task_rq_lock().
> I know it's not pretty, but I'm not sure how your testing wouldn't
> have blown up on the lockdep_assert_held() in select_task_rq().
> 
> Trying a simplified version (without the guard) locally leaves my
> console filled with splats:
>   WARNING: CPU: 20 PID: 0 at kernel/sched/core.c:3746 select_task_rq+0xdc/0x120
> 
> But let me know if there's something I'm being daft about.

Only one being daft was me since I forgot the whole pi_lock dependency
for select_task_rq()! I'll start running with lockdep in my testing to
spot these terrible suggestions early. Sorry for the noise and thanks
a ton for testing that!

-- 
Thanks and Regards,
Prateek