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From: Thomas Gleixner <tglx@linutronix.de>
To: Tejun Heo <htejun@gmail.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>,
	Matt Fleming <matt@console-pimps.org>,
	Oleg Nesterov <oleg@redhat.com>,
	linux-kernel@vger.kernel.org, Tony Luck <tony.luck@intel.com>,
	akpm@linux-foundation.org, torvalds@linux-foundation.org
Subject: Re: [RFC][PATCH 0/5] Signal scalability series
Date: Mon, 3 Oct 2011 15:56:26 +0200 (CEST)	[thread overview]
Message-ID: <alpine.LFD.2.02.1110031544340.1489@ionos> (raw)
In-Reply-To: <20111003013855.GG31799@mtj.dyndns.org>

On Sun, 2 Oct 2011, Tejun Heo wrote:
> On Sat, Oct 01, 2011 at 03:03:29PM +0200, Peter Zijlstra wrote:
> > On Sat, 2011-10-01 at 11:16 +0100, Matt Fleming wrote:
> > > I also think Thomas/Peter mentioned something about latency in
> > > delivering timer signals because of contention on the per-process
> > > siglock. They might have some more details on that. 
> > 
> > Right, so signal delivery is O(nr_threads), which precludes being able
> > to deliver signals from hardirq context, leading to lots of ugly in -rt.
> 
> Signal delivery is O(#threads)?  Delivery of fatal signal is of course
> but where do we walk all threads during non-fatal signal deliveries?
> What am I missing?

Delivery of any process wide signal can result in an O(thread) walk to
find a valid target. That's true for user space originated and kernel
space originated (e.g. posix timers) signals.
 
> > Breaking up the multitude of uses of siglock certainly seems worthwhile
> > esp. if it also allows for a cleanup of the horrid mess called
> > signal_struct (which really should be called process_struct or so).
> > 
> > And yes, aside from that the siglock can be quite contended because its
> > pretty much the one lock serializing all of the process wide state.
> 
> Hmmm... can you please be a bit more specific?  I personally has never
> seen a case where siglock becomes a problem and IIUC Matt also doesn't

Signal heavy applications suffer massivly from sighand->siglock
contention. sighand->siglock protects the world and some more and Matt
has explained it quite proper. And we have rather large code pathes
covered by it (posix-cpu-timers are the worst of all).

> have actual use case at hand.  Given the fragile nature of this part
> of kernel, it would be nice to know what the return is.

The return is finer grained locking and in the end a faster signal
delivery path which benefits everyone as we do not burden a random
interrupted task with the convoluted signal delivery because we want
to burden the task using signals with it.

Thanks,

	tglx

  reply	other threads:[~2011-10-03 13:56 UTC|newest]

Thread overview: 38+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2011-09-30 15:12 [RFC][PATCH 0/5] Signal scalability series Matt Fleming
2011-09-30 15:12 ` [RFC][PATCH 1/5] signal: Document signal locking rules Matt Fleming
2011-09-30 15:12 ` [RFC][PATCH 2/5] signal: Add rwlock to protect sighand->action Matt Fleming
2011-09-30 15:25   ` Peter Zijlstra
2011-09-30 15:56   ` Matt Fleming
2011-09-30 15:12 ` [RFC][PATCH 3/5] signal: Reduce sighand->siglock hold time in get_signal_to_deliver() Matt Fleming
2011-09-30 15:12 ` [RFC][PATCH 4/5] signal: Add signal->ctrl_lock for job control Matt Fleming
2011-09-30 15:30   ` Peter Zijlstra
2011-09-30 15:36     ` Matt Fleming
2011-09-30 15:12 ` [RFC][PATCH 5/5] signal: Split siglock into shared_siglock and per-thread siglock Matt Fleming
2011-09-30 16:52 ` [RFC][PATCH 0/5] Signal scalability series Oleg Nesterov
2011-09-30 18:54   ` Oleg Nesterov
2011-09-30 20:00   ` Matt Fleming
2011-09-30 23:56     ` Tejun Heo
2011-10-01 10:16       ` Matt Fleming
2011-10-01 13:03         ` Peter Zijlstra
2011-10-03  1:38           ` Tejun Heo
2011-10-03 13:56             ` Thomas Gleixner [this message]
2011-10-04  7:37               ` Tejun Heo
2011-10-03 13:07           ` Oleg Nesterov
2011-10-03 15:22             ` Peter Zijlstra
2011-10-04 17:14               ` Oleg Nesterov
2011-10-04 17:52                 ` Oleg Nesterov
2011-10-04 17:54                 ` Linus Torvalds
2011-10-04 18:13                   ` Oleg Nesterov
2011-10-03 13:16     ` Oleg Nesterov
2011-10-04  8:56       ` Matt Fleming
2011-10-04 17:29         ` Oleg Nesterov
2011-09-30 22:30 ` Andi Kleen
2011-10-01  9:35   ` Matt Fleming
2011-10-03 15:28     ` Linus Torvalds
2011-10-03 15:43       ` Matt Fleming
2011-10-03 16:35         ` Oleg Nesterov
2011-10-03 20:58           ` Matt Fleming
2011-10-03 21:45             ` Linus Torvalds
2011-10-03 22:13             ` Thomas Gleixner
2011-10-04  8:20               ` Matt Fleming
2011-10-04 17:39               ` Oleg Nesterov

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