On Sat, Mar 29, 2025 at 08:28:21PM +0100, Mateusz Guzik wrote:
> It is being read surprisingly often (e.g., by mkdir, ls and even sed!).
>
> This is lock-protected pointer chasing over a linked list to pay for
> sprintf for every fs (32 on my boxen).
>
> Instead cache the result.
>
> While here mark the file as permanent to avoid atomic refs on each op
> (which can also degrade to taking a spinlock).
>
> open+read+close cycle single-threaded (ops/s):
> before: 450929
> after: 982308 (+117%)
>
> Here the main bottleneck is memcg.
>
> open+read+close cycle with 20 processes (ops/s):
> before: 578654
> after: 3163961 (+446%)
>
> The main bottleneck *before* is spinlock acquire in procfs eliminated by
> marking the file as permanent. The main bottleneck *after* is the
> spurious lockref trip on open.
>
> The file looks like a sterotypical C from the 90s, right down to an
> open-code and slightly obfuscated linked list. I intentionally did not
> clean up any of it -- I think the file will be best served by a Rust
> rewrite when the time comes.
>
> Signed-off-by: Mateusz Guzik <mjguzik@xxxxxxxxx>
> ---
> + pde = proc_create_single("filesystems", 0, NULL, filesystems_proc_show);
> + proc_make_permanent(pde);
> return 0;
This all looks good enough for me especially if it's really that much of
a bottleneck for some workloads. But the above part is broken because
proc_create_single() may return NULL afair and that means
proc_make_permanent()->pde_make_permanent() will NULL-deref.
I'll fix that up locally.
