lists.openwall.net   lists  /  announce  owl-users  owl-dev  john-users  john-dev  passwdqc-users  yescrypt  popa3d-users  /  oss-security  kernel-hardening  musl  sabotage  tlsify  passwords  /  crypt-dev  xvendor  /  Bugtraq  Full-Disclosure  linux-kernel  linux-netdev  linux-ext4  linux-hardening  linux-cve-announce  PHC 
Open Source and information security mailing list archives
 
Hash Suite: Windows password security audit tool. GUI, reports in PDF.
[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Date: Tue, 26 Mar 2024 11:55:12 +0000
From: Jonthan Haslam <jonathan.haslam@...il.com>
To: Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc: Ingo Molnar <mingo@...nel.org>, linux-trace-kernel@...r.kernel.org, 
	andrii@...nel.org, bpf@...r.kernel.org, rostedt@...dmis.org, mhiramat@...nel.org, 
	Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>, 
	Arnaldo Carvalho de Melo <acme@...nel.org>, Namhyung Kim <namhyung@...nel.org>, 
	Mark Rutland <mark.rutland@....com>, Alexander Shishkin <alexander.shishkin@...ux.intel.com>, 
	Jiri Olsa <jolsa@...nel.org>, Ian Rogers <irogers@...gle.com>, 
	Adrian Hunter <adrian.hunter@...el.com>, linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] uprobes: reduce contention on uprobes_tree access

> > > Have you considered/measured per-CPU RW semaphores?
> >
> > No I hadn't but thanks hugely for suggesting it! In initial measurements
> > it seems to be between 20-100% faster than the RW spinlocks! Apologies for
> > all the exclamation marks but I'm very excited. I'll do some more testing
> > tomorrow but so far it's looking very good.
> >
> 
> Documentation ([0]) says that locking for writing calls
> synchronize_rcu(), is that right? If that's true, attaching multiple
> uprobes (including just attaching a single BPF multi-uprobe) will take
> a really long time. We need to confirm we are not significantly
> regressing this. And if we do, we need to take measures in the BPF
> multi-uprobe attachment code path to make sure that a single
> multi-uprobe attachment is still fast.
> 
> If my worries above turn out to be true, it still feels like a first
> good step should be landing this patch as is (and get it backported to
> older kernels), and then have percpu rw-semaphore as a final (and a
> bit more invasive) solution (it's RCU-based, so feels like a good
> primitive to settle on), making sure to not regress multi-uprobes
> (we'll probably will need some batched API for multiple uprobes).
> 
> Thoughts?

Agreed. In the percpu_down_write() path we call rcu_sync_enter() which is
what calls into synchronize_rcu(). I haven't done the measurements yet but
I would imagine this has to regress probe attachment, at least in the
uncontended case. Of course, reads are by far the dominant mode here but
we probably shouldn't punish writes excessively. I will do some
measurements to quantify the write penalty here.

I agree that a batched interface for probe attachment is needed here. The
usual mode of operation for us is that we have a number of USDTs (uprobes)
in hand and we want to enable and disable them in one shot. Removing the
need to do multiple locking operations is definitely an efficiency
improvement that needs to be done. Tie that together with per-CPU RW
semaphores and this should scale extremely well in both a read and write
case.

Jon.

Powered by blists - more mailing lists

Powered by Openwall GNU/*/Linux Powered by OpenVZ