Re: [PATCH 0/2] sched/psi: fix trigger window arithmetic

[Tao Cui <cui.tao@linux.dev>]

在 2026/7/27 14:10, Suren Baghdasaryan 写道:
> On Thu, Jul 23, 2026 at 9:12 PM Tao Cui <cui.tao@linux.dev> wrote:
>>
>> From: Tao Cui <cuitao@kylinos.cn>
>>
>> Two narrow numeric bugs in the PSI trigger-window arithmetic, both from
>> the original psi monitor (0e94682b73bf) and both only visible for trigger
>> windows larger than ~4.29s (UINT32_MAX ns).
>>
>> 1/2 window_update() estimates the stall growth inside a partially-elapsed
>>    window as `prev_growth * remaining / size`.  `remaining` is a u32 but
>>    win->size is a u64 and may be up to 10s (1e10 ns), so for large windows
>>    it truncates and the interpolation collapses; `prev_growth * remaining`
>>    can also overflow u64.  Use a u64 `remaining` and mul_u64_u64_div_u64().
>>
>> 2/2 psi_trigger_create() scales window/threshold us->ns with
>>    `* NSEC_PER_USEC`.  NSEC_PER_USEC is `long`, so on 32-bit the multiply
>>    wraps for multi-second windows (a 10s window is stored as ~1.41s).
>>    Cast to u64 first.
>>
>> Neither path is the scheduler hot path (update_triggers() runs at
>> trigger-evaluation rate), and small-window triggers are numerically
>> unchanged.
>>
>> Demo of 1/2 -- 10s window, previous window was 100% stall, polled each
>> 1s, trigger "some 5s 10s".  Estimated growth (seconds) inside the window:
>>
>>     elapsed  current(u32)  fixed(u64)
>>     1        1             10
>>     2        2             10
>>     3        3             10
>>     4        5             10
>>
>> With the current code the trigger waits for the actual stall to reach the
>> 5s threshold (~4s in) instead of firing once the run-rate predicts it
>> (~1s in): a ~3s delay on the first window of a stall.
> 
> This looks like a duplicate of the series posted by your collegue at
> https://lore.kernel.org/all/20260717102824.985950-1-guopeng.zhang@linux.dev/
> 
Hi Suren,

You're right -- this duplicates Guopeng's series. He and I found this
issue together a while back, and his submission already covers it, so
I'm withdrawing mine. Sorry for the extra noise.

Thanks,
Tao
>>
>> Tao Cui (2):
>>   sched/psi: fix trigger window growth interpolation for large windows
>>   sched/psi: convert trigger window/threshold to ns in u64
>>
>>  kernel/sched/psi.c | 9 +++++----
>>  1 file changed, 5 insertions(+), 4 deletions(-)
>>
>> --
>> 2.43.0
>>