Messages in this thread Patch in this message | | | Subject | Re: [PATCH 00/21] mm: Preemptibility -v6 | From | Peter Zijlstra <> | Date | Wed, 19 Jan 2011 18:10:39 +0100 |
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On Mon, 2011-01-17 at 23:12 -0800, Hugh Dickins wrote:
> However, there's one more-than-cleanup that I think you will need to add: > the ZAP_BLOCK_SIZE zap_work stuff is still there, but I think it needs > to be removed now, with the need_resched() and other checks moved down > from unmap_vmas() to inside the pagetable spinlock in zap_pte_range(). > > Because you're now accumulating more work than ever in the mmu_gather's > buffer, and the more so with the 20/21 extended list: but this amounts > to a backlog of work which will *usually* be done at the tlb_finish_mmu, > but when memory is low (no extra buffers) may need flushing earlier - > as things stand, while holding the pagetable spinlock, so introducing > a large unpreemptible latency under those conditions. > > I believe that along with the need_resched() check moved inside > zap_pte_range(), you need to check if the mmu_gather buffer is full, > and if so drop pagetable spinlock while you flush it. Hmm, but if > it's extensible, then it wasn't full: I've not worked out how this > would actually fit together.
Very good point!! I'll work on this, I'll probably do a few of those cleanups previously left undone too, I'm seriously doubting the usefulness of the whole restart_addr muck now that its preemptible.
> (I also believe that when memory is low, we *ought* to be freeing up > the pages sooner: perhaps all the GFP_ATOMICs should be GFP_NOWAITs.)
Agreed, I've moved everything to: GFP_NOWAIT | __GFP_NOWARN.
> I found patch ordering a bit odd: I'm going to comment on them in > what seems a more natural ordering to me: if Andrew folds your 00 > comments into 01 as he usually does, then I'd rather see them on the > main preemptible mmu_gather patch, than on reverting some anon_vma > annotations!
Shouldn't we simply ask for better changelogs instead of Andrew doing that? That said, I do like your order better, so did indeed reorder as you suggest.
> And with anon_vma->lock already nested inside i_mmap_lock, > I think the anon_vma mods are secondary, and can just follow after. > > 08/21 mm-preemptible_mmu_gather.patch > Acked-by: Hugh Dickins <hughd@google.com> > But I'd prefer __tlb_alloc_pages() be named __tlb_alloc_page(), > and think it should pass __GFP_NOWARN with its GFP_ATOMIC (same > remark would apply in several other patches too).
Did the rename, and like mentioned, switched to GFP_NOWAIT | __GFP_NOWARN for everything.
> 09/21 powerpc-preemptible_mmu_gather.patch > I'll leave Acking to Ben, but it looked okay so far as I could tell. > I worry how much (unpreemptible) work happens in __flush_tlb_pending > in __switch_to, whether PPC64_TLB_BATCH_NR 192 ought to be smaller > now (I wonder where 192 came from originally); move the _TLF_LAZY_MMU > block below _switch() to after the local_irq_restore(flags)? > The mods to hpte_need_flush() look like what we need in 2.6.37-stable > to keep CONFIG_DEBUG_PREEMPT vfree() quiet, perhaps should be separated > out - but perhaps they're inappropriate and Ben has another fix in mind.
I'll await Ben's answer on this, but yeah, he might consider tuning the 192.
> 10/21 sparc-preemptible_mmu_gather.patch > Similarly, looked okay so far as I could tell, and this one was > already doing flush_tlb_pending in switch_to; more of the 192 > (not from you, of course). tlb_batch_add() has some commented-out > (tb->fullmm) code that you probably meant to come back to. > mm/init_32.c still has DEFINE_PER_CPU(struct mmu_gather, mmu_gathers).
Ah, right.. XXX
> 11/21 s390-preemptible_mmu_gather.patch > I'd prefer __tlb_alloc_page(), with __GFP_NOWARN as suggested above. > mm/pgtable.c still has DEFINE_PER_CPU(struct mmu_gather, mmu_gathers).
Martin, while doing the below DEFINE_PER_CPU removal I saw you had a bunch of RCU table removal thingies in arch/s390/mm/pgtable.c, could s390 use the generic bits like sparc and powerpc (see patch 16)?
> 12/21 arm-preemptible_mmu_gather.patch > 13/21 sh-preemptible_mmu_gather.patch > 14/21 um-preemptible_mmu_gather.patch > 15/21 ia64-preemptible_mmu_gather.patch > All straightforward, but DEFINE_PER_CPU(struct mmu_gather, mmu_gathers) > still to be removed from these and other arches.
I've added a patch removing all the DEFINE_PER_CPU(struct mmu_gather, mmu_gathers) thingies.
One thing I was wondering about, should I fold all these patches into one big patch to improve bisectability? Because after the first patch all !generic-tlb archs won't compile anymore due to the mm/* changes.
> 16/21 mm_powerpc-move_the_rcu_page-table_freeing_into.patch > Seems good, prefer Ben and Dave to Ack. "copmletion" -> "completion".
Fixed the typo, thanks!
> 18/21 mutex-provide_mutex_is_contended.patch > I suppose so, though if we use it in the truncate path, then we are > stuck with the vm_truncate_count stuff I'd rather hoped would go away; > but I guess you're right, that if we did spin_needbreak/need_lockbreak > before, then we ought to do this now - though I suspect I only added > it because I had to insert a resched-point anyway, and it seemed a good > idea at the time to check lockbreak too since that had just been added.
Since its now preemptable we might consider simply removing that. I simply wanted to keep the changes to a minimum for now.
> 19/21 mm-convert_i_mmap_lock_and_anon_vma-_lock_to_mutexes.patch > I suggest doing just the i_mmap_lock->mutex conversion at this point. > Acked-by: Hugh Dickins <hughd@google.com> > except that in the past we have renamed a lock when we've done this > kind of conversion, so I'd expect i_mmap_mutex throughout now. > Or am I just out of date? I don't feel very strongly about it.
Done, split the conversion and did the s/_lock/_mutex/ thing.
> 20/21 mm-extended_batches_for_generic_mmu_gather.patch > Acked-by: Hugh Dickins <hughd@google.com> > though it struck me as overdesign at first: I guess Nick wanted it > because he had an implementation that used the pagetables themselves, > hence an assured supply of these buffers. tlb_finish_mmu(), and > perhaps others, looking rather too big for inline by this stage.
Yeah, they are a bit beefy, maybe I should move some of that into mm/memory.c.
> 04/21 mm-rename_drop_anon_vma_to_put_anon_vma.patch > Acked-by: Hugh Dickins <hughd@google.com> > but (if you don't mind: leave it to me if you prefer) in mm/ksm.c > please just remove wrappers hold_anon_vma() and ksm_put_anon_vma(): > they had a point when they originated the refcount but no point now. > Note there are now two places to update in mm/migrate.c in 38-rc1.
Done, zapped those wrappers.
> 05/21 mm-move_anon_vma_ref_out_from_under_config_ksm.patch > Acked-by: Hugh Dickins <hughd@google.com> > but you shouldn't need to touch mm/migrate.c again here with 38-rc1. > Didn't you end up double-decrementing refcount in the huge_page case?
I'm afraid I need a little help here, what huge_page case? I tried applying this comment to both patches 5 and 6, but failed to find a huge_page case..
> 06/21 mm-simplify_anon_vma_refcounts.patch > Acked-by: Hugh Dickins <hughd@google.com> > except page_get_anon_vma() is being declared in rmap.h a patch early, > and you shouldn't need to touch mm/ksm.c again here with 38-rc1. > Did wonder if __put_anon_vma() is right to put anon_vma->root *before* > freeing anon_vma, but suppose your not_zero strictness makes it safe.
It seemed like the natural order to do things, release the reference we hold on ->root right before we free ourselves.
The race you're worried about is the page_lock_anon_vma() where we access ->root? Afaict that's ok because we check page_mapped() and decrementing that should be done _before_ the last put_anon_vma(), otherwise that function is already racy.
> 07/21 mm-use_refcounts_for_page_lock_anon_vma.patch > Acked-by: Hugh Dickins <hughd@google.com> > but here I'm expecting you to use your page_get_anon_vma() in > mm/migrate.c too, to replace my 38-rc1 lock/get/unlock sequences.
done
> Second page_mapped() test in page_get_anon_vma(): remove "goto out;" > from that block, it's already reached "out".
Paranoia on my side, done.
> In patch description, > didn't understand "for each of convertion": "for sake of conversion"?
Uhm.. yeah my brain must have slipped there or something, not quite sure what I meant, let me make that:
"This is done to prepare for the conversion of anon_vma from spinlock to mutex."
> This brings us to a nice point, ready for the lock->mutex conversion: > the only defect being the doubled atomics in page_(un)lock_anon_vma.
Yeah, that double atomic is sadness, you've seen what I've come up with to avoid that..
> 19/21 mm-convert_i_mmap_lock_and_anon_vma-_lock_to_mutexes.patch > I suggest doing the anon_vma lock->mutex conversion separately here. > Acked-by: Hugh Dickins <hughd@google.com> > except that in the past we have renamed a lock when we've done this > kind of conversion, so I'd expect anon_vma->mutex throughout now. > Or am I just out of date? I don't feel very strongly about it.
Done.. however:
Index: linux-2.6/include/linux/huge_mm.h =================================================================== --- linux-2.6.orig/include/linux/huge_mm.h +++ linux-2.6/include/linux/huge_mm.h @@ -91,12 +91,8 @@ extern void __split_huge_page_pmd(struct #define wait_split_huge_page(__anon_vma, __pmd) \ do { \ pmd_t *____pmd = (__pmd); \ - spin_unlock_wait(&(__anon_vma)->root->lock); \ - /* \ - * spin_unlock_wait() is just a loop in C and so the \ - * CPU can reorder anything around it. \ - */ \ - smp_mb(); \ + anon_vma_lock(__anon_vma); \ + anon_vma_unlock(__anon_vma); \ BUG_ON(pmd_trans_splitting(*____pmd) || \ pmd_trans_huge(*____pmd)); \ } while (0) Andrea, is that smp_mb() simply to avoid us doing anything before the lock is free? Why isn't there an mb() before to ensure nothing leaks past it from the other end?
> 21/21 mm-optimize_page_lock_anon_vma_fast-path.patch > I certainly see the call for this patch, I want to eliminate those > doubled atomics too. This appears correct to me, and I've not dreamt > up an alternative; but I do dislike it, and I suspect you don't like > it much either. I'm ambivalent about it, would love a better patch.
Like said, I fully agree with that sentiment, just haven't been able to come up with anything saner :/ Although I can optimize the __put_anon_vma() path a bit by doing something like:
if (mutex_is_locked()) { anon_vma_lock(); anon_vma_unlock(); } But I bet that wants a barrier someplace and my head hurts..
> sparc64-Kill_page_table_quicklists.patch > sparc64-Use_RCU_page_table_freeing.patch > sparc64-Add_support_for__PAGE_SPECIA.patch > sparc64-Implement_get_user_pages_fast.patch > I did not spend very long looking at these, none of my business really! > but did notice one thing I didn't like, that pte_special() is declared > unsigned long in the third, whereas int in every other architecture. I > think it should follow the ia64-style there, use != 0 to return an int.
Dave, do you want me to make those changes, or will you once the rest of this stuff makes it upstream?
> A few checkpatch warnings, many of which I don't particularly agree with - > though I do get annoyed by comments going over 80-cols without any need!
Agreed, although I didn't spot any comments crossing the 80-column boundary.
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