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

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Message-ID: <50a5ebdb-4107-26cc-a2f6-da551d99ff38@kernel.dk>
Date:   Tue, 3 Jan 2023 07:25:26 -0700
From:   Jens Axboe <axboe@...nel.dk>
To:     "Russell King (Oracle)" <linux@...linux.org.uk>,
        Hui Tang <tanghui20@...wei.com>
Cc:     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 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)

-- 
Jens Axboe