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SubjectRe: [bug-report] possible performance problem in ret_to_user_from_irq
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On 2023/1/3 22:59, Jens Axboe wrote:
> On 1/3/23 7:34?AM, Russell King (Oracle) wrote:
>> On Tue, Jan 03, 2023 at 07:25:26AM -0700, Jens Axboe wrote:
>>> On 1/3/23 3:06?AM, Russell King (Oracle) wrote:
>>>> On Mon, Dec 26, 2022 at 04:45:20PM +0800, Hui Tang wrote:
>>>>> hi folks.
>>>>>
>>>>> I found a performance problem which is introduced by commit
>>>>> 32d59773da38 ("arm: add support for TIF_NOTIFY_SIGNAL").
>>>>> After the commit, any bit in the range of 0..15 will cause
>>>>> do_work_pending() to be invoked. More frequent do_work_pending()
>>>>> invoked possible result in worse performance.
>>>>>
>>>>> Some of the tests I've done? as follows:
>>>>> lmbench test base with patch
>>>>> ./lat_ctx -P 1 -s 0 2 7.3167 11.04
>>>>> ./lat_ctx -P 1 -s 16 2 8.0467 14.5367
>>>>> ./lat_ctx -P 1 -s 64 2 7.8667 11.43
>>>>> ./lat_ctx -P 1 -s 16 16 16.47 18.3667
>>>>> ./lat_pipe -P 1 28.1671 44.7904
>>>>>
>>>>> libMicro-0.4.1 test base with patch
>>>>> ./cascade_cond -E -C 200\
>>>>> -L -S -W -N "c_cond_1" -I 100 286.3333 358
>>>>>
>>>>> When I adjust test bit, the performance problem gone.
>>>>> - movs r1, r1, lsl #16
>>>>> + ldr r2, =#_TIF_WORK_MASK
>>>>> + tst r1, r2
>>>>>
>>>>> Does anyone have a good suggestion for this problem?
>>>>> should just test _TIF_WORK_MASK, as before?
>>>>
>>>> I think it should be fine - but I would suggest re-organising the
>>>> TIF definitions so that those TIF bits that shouldn't trigger
>>>> do_work_pending are not in the first 16 bits.
>>>>
>>>> Note that all four bits in _TIF_SYSCALL_WORK need to stay within
>>>> an 8-bit even-bit-aligned range, so the value is suitable for an
>>>> immediate assembly constant.
>>>>
>>>> I'd suggest moving the TIF definitions for 20 to 19, and 4..7 to
>>>> 20..23, and then 8 to 4.
>>>
>>> Like this?
>>>
>>> diff --git a/arch/arm/include/asm/thread_info.h b/arch/arm/include/asm/thread_info.h
>>> index aecc403b2880..7f092cb55a41 100644
>>> --- a/arch/arm/include/asm/thread_info.h
>>> +++ b/arch/arm/include/asm/thread_info.h
>>> @@ -128,15 +128,16 @@ extern int vfp_restore_user_hwstate(struct user_vfp *,
>>> #define TIF_NEED_RESCHED 1 /* rescheduling necessary */
>>> #define TIF_NOTIFY_RESUME 2 /* callback before returning to user */
>>> #define TIF_UPROBE 3 /* breakpointed or singlestepping */
>>> -#define TIF_SYSCALL_TRACE 4 /* syscall trace active */
>>> -#define TIF_SYSCALL_AUDIT 5 /* syscall auditing active */
>>> -#define TIF_SYSCALL_TRACEPOINT 6 /* syscall tracepoint instrumentation */
>>> -#define TIF_SECCOMP 7 /* seccomp syscall filtering active */
>>> -#define TIF_NOTIFY_SIGNAL 8 /* signal notifications exist */
>>> +#define TIF_NOTIFY_SIGNAL 4 /* signal notifications exist */
>>>
>>> #define TIF_USING_IWMMXT 17
>>> #define TIF_MEMDIE 18 /* is terminating due to OOM killer */
>>> -#define TIF_RESTORE_SIGMASK 20
>>> +#define TIF_RESTORE_SIGMASK 19
>>> +#define TIF_SYSCALL_TRACE 20 /* syscall trace active */
>>> +#define TIF_SYSCALL_AUDIT 21 /* syscall auditing active */
>>> +#define TIF_SYSCALL_TRACEPOINT 22 /* syscall tracepoint instrumentation */
>>> +#define TIF_SECCOMP 23 /* seccomp syscall filtering active */
>>> +
>>>
>>> #define _TIF_SIGPENDING (1 << TIF_SIGPENDING)
>>> #define _TIF_NEED_RESCHED (1 << TIF_NEED_RESCHED)
>>
>> Yep, LGTM, thanks.
>
> Hui Tang, can you give it a whirl? Just checked and it applies to
> 5.10-stable as well, just with a slight offset.

Okay, I'll test it today.

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