Date:	Fri, 02 May 2008 18:13:05 +0200
From:	Petr Tesarik <ptesarik@...e.cz>
To:	Björn Steinbrink <B.Steinbrink@....de>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Oleg Nesterov <oleg@...sign.ru>, linux-kernel@...r.kernel.org,
	Roland McGrath <roland@...hat.com>,
	Frank Mayhar <fmayhar@...gle.com>
Subject: Re: CPU POSIX timers livelock

On Fri, 2008-05-02 at 17:21 +0200, Björn Steinbrink wrote:
> [Added Roland McGrath and Frank Mayhar to Cc:, as this sounds similar
> enough to what has been discussed here http://lkml.org/lkml/2008/2/6/505]

Yes, I've just now found the thread too, read it, and I think this is
just another case where the current implementation does not scale.

Was there any followup to the patch posted on the 7th of March? The
interesting discussion seems to be interrupted there. :(

Petr Tesarik

> On 2008.05.02 17:05:41 +0200, Petr Tesarik wrote:
> > Hello,
> > 
> > there seems to be a possible livelock in the CPU timers (actually, we've
> > experienced it once already). The analysis lead to the conclusion that
> > fixing this properly will be a bit harder.
> > 
> > Problem Statement
> > 
> >  1. Any process can set ITIMER_PROF as short as 1 timer tick
> >  2. If the process is CPU-bound (e.g. a number-crunching application),
> >     the timer will expire with every timer tick
> >  3. If the process has N threads and each thread runs on its own
> >     CPU, the timer will expire N times per timer tick
> > 
> > Now, this can become quite interesting on a larger system. For instance,
> > with 128 CPUs and a (conservative) setting of HZ 300, any process can
> > effectively ask to be sent a SIGPROF every 26 usec (1/300/128 s). Pretty
> > fast, but it can get worse if you consider the pitfalls of a NUMA
> > architecture.
> > 
> > Whenever the timer expires, a new expiration time must be computed for
> > each thread in the thread group. The values are stored in task_struct of
> > each thread, and IIRC those are kept local to the CPU where the thread
> > is running (quite logically). Put in another way, walking all threads
> > means touching remote memory except for the few task_struct which are
> > local to the recomputing CPU. In the 128-CPU example, if each node has 2
> > CPUs (think Altix), only 2 accesses are to the local node and 126
> > accesses are to remote memory. These are already quite expensive, but it
> > gets even worse. The timer interrupt is generated locally for each CPU,
> > so if the process is spread over all of them (see above), all CPUs
> > recompute the timer (hint: write access), thus constantly invalidating
> > local caches of the remote memory.
> > 
> > And those computations cannot run in parallel, of course, because
> > everything is serialized on sighand->siglock (see run_posix_cpu_timers).
> > So, each time one CPU finishes walking the threads, there's already a
> > queue of all the other CPUs waiting to do the same again. For this kind
> > of load, those 26 usec (approx. 41600 CPU cycles on a 1.6 GHz CPU) may
> > be well insufficient. Even more so, if somebody else occasionally takes
> > write lock on tasklist_lock. It may so happen that the system spends all
> > CPU cycles re-computing the profiling timers over and over again and
> > never make any progress. Note this is partly caused by the fact that the
> > time needed to handle the profiling timer is itself also counted to the
> > profiling time.
> > 
> > This simply does not scale for large systems (and I was not even
> > considering those really large 1024p ones).
> > 
> > Ideas
> > 
> > Any ideas for improvement? Obviously, storing per-process expiration
> > times in one place instead of dividing the interval by the number of
> > currently running threads would help here. But it would spoil the fast
> > path (when there are no timers at all). Avoiding that multiple threads
> > re-compute the same values might also help (although I've got no idea
> > how to implement this).
> > 
> > Any more comments (before I start hacking something you'll NAK)?
> > 
> > BTW did POSIX really mean to allow an indefinitely small interval for
> > the SIGPROF?
> > 
> > Petr Tesarik
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