linux-kernel - Re: [bug-report] possible performance problem in ret_to_user_from

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Message-ID: <1ecb9b0c-1103-650a-e32a-93110466b2ae@kernel.dk>
Date:   Tue, 3 Jan 2023 07:59:07 -0700
From:   Jens Axboe <axboe@...nel.dk>
To:     "Russell King (Oracle)" <linux@...linux.org.uk>
Cc:     Hui Tang <tanghui20@...wei.com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [bug-report] possible performance problem in ret_to_user_from_irq

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.

-- 
Jens Axboe