On Tue, Mar 04, 2025 at 07:35:02PM +0100, Mateusz Guzik wrote:
> The stock kernel transitioning the file to no refs held penalizes the
> caller with an extra atomic to block any increments.
>
> For cases where the file is highly likely to be going away this is
> easily avoidable.
>
> In the open+close case the win is very modest because of the following
> problems:
> - kmem and memcg having terrible performance
I thought that was going to be addressed by Vlastimil, i.e., the mm guys
to provide a new memcg api.
> - putname using an atomic (I have a wip to whack that)
> - open performing an extra ref/unref on the dentry (there are patches to
> do it, including by Al. I mailed about them in [1])
> - creds using atomics (I have a wip to whack that)
> - apparmor using atomics (ditto, same mechanism)
>
> On top of that I have a WIP patch to dodge some of the work at lookup
> itself.
>
> All in all there is several % avoidably lost here.
>
> stats colected during a kernel build with:
> bpftrace -e 'kprobe:filp_close,kprobe:fput,kprobe:fput_close* { @[probe] = hist(((struct file *)arg0)->f_ref.refcnt.counter > 0); }'
>
> @[kprobe:filp_close]:
> [0] 32195 |@@@@@@@@@@ |
> [1] 164567 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@|
>
> @[kprobe:fput]:
> [0] 339240 |@@@@@@ |
> [1] 2888064 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@|
>
> @[kprobe:fput_close]:
> [0] 5116767 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@|
> [1] 164544 |@ |
>
> @[kprobe:fput_close_sync]:
> [0] 5340660 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@|
> [1] 358943 |@@@ |
>
>
> 0 indicates the last reference, 1 that there is more.
>
> filp_close is largely skewed because of close_on_exec.
>
> vast majority of last fputs are from remove_vma. I think that code wants
> to be patched to batch them (as in something like fput_many should be
> added -- something for later).
We used to have that for io_uring and got rid of it. The less fput()
primitives the better tbh. But let's see.
>
> [1] https://lore.kernel.org/linux-fsdevel/20250304165728.491785-1-mjguzik@xxxxxxxxx/T/#u
>
> v2:
> - patch filp_close
> - patch failing open
>
> Mateusz Guzik (4):
> file: add fput and file_ref_put routines optimized for use when
> closing a fd
> fs: use fput_close_sync() in close()
> fs: use fput_close() in filp_close()
> fs: use fput_close() in path_openat()
>
> fs/file.c | 75 ++++++++++++++++++++++++++++++----------
> fs/file_table.c | 72 +++++++++++++++++++++++++++-----------
> fs/namei.c | 2 +-
> fs/open.c | 4 +--
> include/linux/file.h | 2 ++
> include/linux/file_ref.h | 1 +
> 6 files changed, 114 insertions(+), 42 deletions(-)
>
> --
> 2.43.0
>
