On Thu, May 30, 2013 at 10:34:53AM +0530, Vineet Gupta wrote:
> On 05/29/2013 11:21 PM, Peter Zijlstra wrote:
> > What about something like this?
> >
> > ---
> > include/asm-generic/tlb.h | 11 ++++++++++-
> > mm/memory.c | 17 ++++++++++++++++-
> > 2 files changed, 26 insertions(+), 2 deletions(-)
> >
> > diff --git a/include/asm-generic/tlb.h b/include/asm-generic/tlb.h
> > index b1b1fa6..651b1cf 100644
> > --- a/include/asm-generic/tlb.h
> > +++ b/include/asm-generic/tlb.h
> > @@ -116,6 +116,7 @@ struct mmu_gather {
> >
> > static inline int tlb_fast_mode(struct mmu_gather *tlb)
> > {
> > +#ifndef CONFIG_PREEMPT
> > #ifdef CONFIG_SMP
> > return tlb->fast_mode;
> > #else
> > @@ -124,7 +125,15 @@ static inline int tlb_fast_mode(struct mmu_gather *tlb)
> > * and page free order so much..
> > */
> > return 1;
> > -#endif
> > +#endif /* CONFIG_SMP */
> > +#else /* CONFIG_PREEMPT */
> > + /*
> > + * Since mmu_gather is preemptible, preemptible kernels are like SMP
> > + * kernels, we must batch to make sure we invalidate TLBs before we
> > + * free the pages.
> > + */
> > + return 0;
> > +#endif /* CONFIG_PREEMPT */
> > }
>
> So this adds the page batching logic to small/simpler UP systems - but it's
> necessary evil :-(
Well, only if you have CONFIG_PREEMPT=y, at which point UP isn't really
_that_ simply anymore really.
> > void tlb_gather_mmu(struct mmu_gather *tlb, struct mm_struct *mm, bool fullmm);
> > diff --git a/mm/memory.c b/mm/memory.c
> > index 6dc1882..e915af2 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -384,6 +384,21 @@ void tlb_remove_table(struct mmu_gather *tlb, void *table)
> >
> > #endif /* CONFIG_HAVE_RCU_TABLE_FREE */
> >
> > +static inline void cond_resched_tlb(struct mmu_gather *tlb)
> > +{
> > +#ifndef CONFIG_PREEMPT
> > + /*
> > + * For full preempt kernels we must do regular batching like
> > + * SMP, see tlb_fast_mode(). For !PREEMPT we can 'cheat' and
> > + * do a flush before our voluntary 'yield'.
> > + */
> > + if (need_resched()) {
>
> This is really neat: w/o this check, a @fullmm flush (exit/execve) would have
> suffered multiple full TLB flushes in the loop, now you do that only if a
> scheduling was needed - meaning only in the case when we have the potential race
> condition which Max was seeing. Cool !
Glad you like it ;-)
> > + tlb_flush_mmu(tlb);
> > + cond_resched();
> > + }
> > +#endif
> > +}
> > +
> > /*
> > * If a p?d_bad entry is found while walking page tables, report
> > * the error, before resetting entry to p?d_none. Usually (but
> > @@ -1264,7 +1279,7 @@ static inline unsigned long zap_pmd_range(struct mmu_gather *tlb,
> > goto next;
> > next = zap_pte_range(tlb, vma, pmd, addr, next, details);
> > next:
> > - cond_resched();
> > + cond_resched_tlb(tlb);
> > } while (pmd++, addr = next, addr != end);
> >
> > return addr;
>
> BTW, since we are on the topic, it seems that we are missing tlb_fast_mode() in
> one spot - unless it is tied to rcu table free stuff.
>
> -------------->
> From: Vineet Gupta <vgupta@xxxxxxxxxxxx>
> Date: Thu, 30 May 2013 10:25:30 +0530
> Subject: [PATCH] mm: tlb_fast_mode check missing in tlb_finish_mmu()
>
> Signed-off-by: Vineet Gupta <vgupta@xxxxxxxxxxxx>
> ---
> mm/memory.c | 3 +++
> 1 file changed, 3 insertions(+)
>
> diff --git a/mm/memory.c b/mm/memory.c
> index d9d5fd9..569ffe1 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -269,6 +269,9 @@ void tlb_finish_mmu(struct mmu_gather *tlb, unsigned long
> start, unsigned long e
> /* keep the page table cache within bounds */
> check_pgt_cache();
>
> + if (tlb_fast_mode(tlb))
> + return;
> +
> for (batch = tlb->local.next; batch; batch = next) {
> next = batch->next;
> free_pages((unsigned long)batch, 0);
Yes I think that is possible. It would shrink the code a little when
fast_mode was unconditionally 1 -- ie. simple UP ;-).
--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@xxxxxxxxx. For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>