lkml.org 
[lkml]   [2012]   [Dec]   [18]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    SubjectRe: [PATCH] mm: cond_resched in tlb_flush_mmu to fix soft lockups on !CONFIG_PREEMPT
    On Tue 18-12-12 14:02:19, Andrew Morton wrote:
    > On Tue, 18 Dec 2012 17:11:28 +0100
    > Michal Hocko <mhocko@suse.cz> wrote:
    >
    > > Since e303297 (mm: extended batches for generic mmu_gather) we are batching
    > > pages to be freed until either tlb_next_batch cannot allocate a new batch or we
    > > are done.
    > >
    > > This works just fine most of the time but we can get in troubles with
    > > non-preemptible kernel (CONFIG_PREEMPT_NONE or CONFIG_PREEMPT_VOLUNTARY) on
    > > large machines where too aggressive batching might lead to soft lockups during
    > > process exit path (exit_mmap) because there are no scheduling points down the
    > > free_pages_and_swap_cache path and so the freeing can take long enough to
    > > trigger the soft lockup.
    > >
    > > The lockup is harmless except when the system is setup to panic on
    > > softlockup which is not that unusual.
    > >
    > > The simplest way to work around this issue is to explicitly cond_resched per
    > > batch in tlb_flush_mmu (1020 pages on x86_64).
    > >
    > > ...
    > >
    > > --- a/mm/memory.c
    > > +++ b/mm/memory.c
    > > @@ -239,6 +239,7 @@ void tlb_flush_mmu(struct mmu_gather *tlb)
    > > for (batch = &tlb->local; batch; batch = batch->next) {
    > > free_pages_and_swap_cache(batch->pages, batch->nr);
    > > batch->nr = 0;
    > > + cond_resched();
    > > }
    > > tlb->active = &tlb->local;
    > > }
    >
    > tlb_flush_mmu() has a large number of callsites (or callsites which
    > call callers, etc), many in arch code. It's not at all obvious that
    > tlb_flush_mmu() is never called from under spinlock?

    free_pages_and_swap_cache calls lru_add_drain which in turn calls
    put_cpu (aka preempt_enable) which is a scheduling point for
    CONFIG_PREEMPT. There are more down the call chain probably. None of
    them for non-preempt kernel.
    --
    Michal Hocko
    SUSE Labs


    \
     
     \ /
      Last update: 2012-12-19 02:01    [W:2.421 / U:0.044 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site