Linus Torvalds 021a160abf
fs: use __fput_sync in close(2) ...
close(2) is a special case which guarantees a shallow kernel stack,
making delegation to task_work machinery unnecessary. Said delegation is
problematic as it involves atomic ops and interrupt masking trips, none
of which are cheap on x86-64. Forcing close(2) to do it looks like an
oversight in the original work.
Moreover presence of CONFIG_RSEQ adds an additional overhead as fput()
-> task_work_add(..., TWA_RESUME) -> set_notify_resume() makes the
thread returning to userspace land in resume_user_mode_work(), where
rseq_handle_notify_resume takes a SMAP round-trip if rseq is enabled for
the thread (and it is by default with contemporary glibc).
Sample result when benchmarking open1_processes -t 1 from will-it-scale
(that's an open + close loop) + tmpfs on /tmp, running on the Sapphire
Rapid CPU (ops/s):
stock+RSEQ: 1329857
stock-RSEQ: 1421667 (+7%)
patched: 1523521 (+14.5% / +7%) (with / without rseq)
Patched result is the same regardless of rseq as the codepath is avoided.
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org >
Signed-off-by: Christian Brauner <brauner@kernel.org >
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021a160abf