Re: [PATCH] kretprobe scalability improvement

[Matt Wu]

On 2021/7/5 AM7:59, Masami Hiramatsu wrote:
> On Sun, 4 Jul 2021 10:16:47 +0100
> Christoph Hellwig <hch@xxxxxxxxxxxxx> wrote:
>
> > Would it make sense to just reuse kernel/bpf/percpu_freelist.c for
> > kretprobes?
>
> Hmm, I don't think so.
> It seems that what Wuqiang proposed is more efficient than the
> percpu_freelist, and it will be less efficient from the viewpoint
> of memory usage because kretprobe freelist manages instance pool
> among all CPUs (which can be unbalanced, sometimes 95% used by one
> core, sometimes used evenly).
>
> Actually, the best solution is to have per-task fixed-size instance
> pool which is shared by all kretprobes (e.g. 4kb/task), because
> the instance makes a "shadow stack" for each task. This may consume
> more memory but is not increased by adding kretprobes, and should be
> scalable.

Yes, per-task pool is the best for scalability.

How about allocating the kretprobe instance just from stack ? The size
of kretprobe instance is very likely to be "small", then most of allocs
could be fed quickly from current stack.

Expanding default kernel stack by 1 page is also an option, but the
impact of memory occupation would be huge, after all the kretprobe is
a rare thing and uncertain to normal threads.

Regards,
Matt Wu