Messages in this thread | | | Date | Fri, 7 Nov 2008 09:35:55 +0100 | From | Ingo Molnar <> | Subject | Re: regression introduced by - timers: fix itimer/many thread hang |
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* Frank Mayhar <fmayhar@google.com> wrote:
> On Thu, 2008-11-06 at 16:08 +0100, Peter Zijlstra wrote: > > On Thu, 2008-11-06 at 09:03 -0600, Christoph Lameter wrote: > > > On Thu, 6 Nov 2008, Peter Zijlstra wrote: > > > > > > > Also, you just introduced per-cpu allocations for each thread-group, > > > > while Christoph is reworking the per-cpu allocator, with one unfortunate > > > > side-effect - its going to have a limited size pool. Therefore this will > > > > limit the number of thread-groups we can have. > > > > > > Patches exist that implement a dynamically growable percpu pool (using > > > virtual mappings though). If the cost of the additional complexity / > > > overhead is justifiable then we can make the percpu pool dynamically > > > extendable. > > > > Right, but I don't think the patch under consideration will fly anyway, > > doing a for_each_possible_cpu() loop on every tick on all cpus isn't > > really healthy, even for moderate sized machines. > > I personally think that you're overstating this. First, the current > implementation walks all threads for each tick, which is simply not > scalable and results in soft lockups with large numbers of threads. > This patch fixes a real bug. Second, this only happens "on every > tick" for processes that have more than one thread _and_ that use > posix interval timers. Roland and I went to some effort to keep > loops like the on you're referring to out of the common paths. > > In any event, while this particular implementation may not be > optimal, at least it's _right_. Whatever happened to "make it > right, then make it fast?"
Well, you pushed the lockup to another place: previously we locked up with enough threads added, now we'll lock up with enough CPUs added.
So ... please get rid of the for-each-cpu loop for good? (Also, the task-exit race needs to be fixed first i guess, before we worry about loops.)
Ingo
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