Messages in this thread | | | Date | Wed, 19 Dec 2012 00:50:42 +0100 | From | Michal Hocko <> | Subject | Re: [PATCH] mm: cond_resched in tlb_flush_mmu to fix soft lockups on !CONFIG_PREEMPT |
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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
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