[bug-report] possible performance problem in ret_to_user_from_irq

[Hui Tang]

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?