Messages in this thread | | | Date | Mon, 15 Nov 2010 20:47:03 -0800 | From | Andrew Morton <> | Subject | Re: [PATCH][RESEND] nommu: yield CPU periodically while disposing large VM |
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On Mon, 15 Nov 2010 08:29:11 -0600 "Steven J. Magnani" <steve@digidescorp.com> wrote:
> On Thu, 2010-11-11 at 18:40 -0800, Andrew Morton wrote: > > On Thu, 11 Nov 2010 14:33:16 -0600 "Steven J. Magnani" <steve@digidescorp.com> wrote: > > > > > --- a/mm/nommu.c 2010-10-21 07:42:23.000000000 -0500 > > > +++ b/mm/nommu.c 2010-10-21 07:46:50.000000000 -0500 > > > @@ -1656,6 +1656,7 @@ SYSCALL_DEFINE2(munmap, unsigned long, a > > > void exit_mmap(struct mm_struct *mm) > > > { > > > struct vm_area_struct *vma; > > > + unsigned long next_yield = jiffies + HZ; > > > > > > if (!mm) > > > return; > > > @@ -1668,6 +1669,11 @@ void exit_mmap(struct mm_struct *mm) > > > mm->mmap = vma->vm_next; > > > delete_vma_from_mm(vma); > > > delete_vma(mm, vma); > > > + /* Yield periodically to prevent watchdog timeout */ > > > + if (time_after(jiffies, next_yield)) { > > > + cond_resched(); > > > + next_yield = jiffies + HZ; > > > + } > > > } > > > > > > kleave(""); > > > [snip] > > cond_resched() is pretty efficient and one second is still > > a very long time. I suspect you don't need the ratelimiting at all? > > Probably not, but the issue was that disposal of "large" VMs can starve > the system. Since these are not the norm (otherwise this would have been > fixed long ago) I was attempting to limit the impact on more > "normal"-sized VMs. Responsiveness is not great with a one-second > ratelimit, and as KOSAKI Motohiro points out this fix won't work on > systems with short watchdog intervals. I assumed that these were not > common. > > As efficient as schedule() may be, it still scares me to call it on > reclaim of every block of memory allocated by a terminating process, > particularly on the relatively slow processors that inhabit NOMMU land.
This is cond_resched(), not schedule()! cond_resched() is just a few instructions, except for the super-rare case where it calls schedule().
> It wasn't obvious to me that it has a quick exit. But since we are > talking about sharing the CPU with other processes perhaps this is only > an issue in an OOM scenario, when fast reclaim might be more important. > > I can certainly respin the patch to call cond_resched() unconditionally > if that's the consensus.
You have a consensus of 1 so far :)
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