Re: [PATCH] tools api fs: make xxx__mountpoint() more scalable

[Jiri Olsa <jolsa@redhat.com>]

On Fri, Mar 27, 2020 at 06:42:21PM -0700, Ian Rogers wrote:
> From: Stephane Eranian <eranian@google.com>
> 
> The xxx_mountpoint() interface provided by fs.c finds
> mount points for common pseudo filesystems. The first
> time xxx_mountpoint() is invoked, it scans the mount
> table (/proc/mounts) looking for a match. If found, it
> is cached. The price to scan /proc/mounts is paid once
> if the mount is found.
> 
> When the mount point is not found, subsequent calls to
> xxx_mountpoint() scan /proc/mounts over and over again.
> There is no caching.
> 
> This causes a scaling issue in perf record with hugeltbfs__mountpoint().
> The function is called for each process found in synthesize__mmap_events().
> If the machine has thousands of processes and if the /proc/mounts has many
> entries this could cause major overhead in perf record. We have observed
> multi-second slowdowns on some configurations.
> 
> As an example on a laptop:
> 
> Before:
> $ sudo umount /dev/hugepages
> $ strace -e trace=openat -o /tmp/tt perf record -a ls
> $ fgrep mounts /tmp/tt
> 285
> 
> After:
> $ sudo umount /dev/hugepages
> $ strace -e trace=openat -o /tmp/tt perf record -a ls
> $ fgrep mounts /tmp/tt
> 1
> 
> One could argue that the non-caching in case the moint point is not found
> is intentional. That way subsequent calls may discover a moint point if
> the sysadmin mounts the filesystem. But the same argument could be made
> against caching the mount point. It could be unmounted causing errors.
> It all depends on the intent of the interface. This patch assumes it
> is expected to scan /proc/mounts once. The patch documents the caching
> behavior in the fs.h header file.

I agree, I don't think we have a code that would intentionaly
make use of the later discovery.. if that's ever needed we
could add function that invalidates the cache

Acked-by:  Jiri Olsa <jolsa@redhat.com>

thanks,
jirka