On Sat, Feb 26, 2022 at 02:32:01PM +0800, Baokun Li wrote:
From: Linus Torvalds torvalds@linux-foundation.org
commit e386dfc56f837da66d00a078e5314bc8382fab83 upstream.
Commit 054aa8d439b9 ("fget: check that the fd still exists after getting a ref to it") fixed a race with getting a reference to a file just as it was being closed. It was a fairly minimal patch, and I didn't think re-checking the file pointer lookup would be a measurable overhead, since it was all right there and cached.
But I was wrong, as pointed out by the kernel test robot.
The 'poll2' case of the will-it-scale.per_thread_ops benchmark regressed quite noticeably. Admittedly it seems to be a very artificial test: doing "poll()" system calls on regular files in a very tight loop in multiple threads.
That means that basically all the time is spent just looking up file descriptors without ever doing anything useful with them (not that doing 'poll()' on a regular file is useful to begin with). And as a result it shows the extra "re-check fd" cost as a sore thumb.
Happily, the regression is fixable by just writing the code to loook up the fd to be better and clearer. There's still a cost to verify the file pointer, but now it's basically in the noise even for that benchmark that does nothing else - and the code is more understandable and has better comments too.
[ Side note: this patch is also a classic case of one that looks very messy with the default greedy Myers diff - it's much more legible with either the patience of histogram diff algorithm ]
Link: https://lore.kernel.org/lkml/20211210053743.GA36420@xsang-OptiPlex-9020/ Link: https://lore.kernel.org/lkml/20211213083154.GA20853@linux.intel.com/ Reported-by: kernel test robot oliver.sang@intel.com Tested-by: Carel Si beibei.si@intel.com Cc: Jann Horn jannh@google.com Cc: Miklos Szeredi mszeredi@redhat.com Signed-off-by: Linus Torvalds torvalds@linux-foundation.org Signed-off-by: Baokun Li libaokun1@huawei.com
fs/file.c | 73 +++++++++++++++++++++++++++++++++++++++++++------------ 1 file changed, 57 insertions(+), 16 deletions(-)
All now queued up, thanks for the backports!
greg k-h