On Tue, 24 Jul 2007 10:38:55 +0200 Ingo Molnar <[email protected]> wrote:
>
> the patch below has been tested in -rt for a long time - lets get it
> upstream please. It fixes some bad latencies on PREEMPT too.
>
> Ingo
>
> ----------------------->
> From: Hugh Dickins <[email protected]>
> Subject: mm: reduce pagetable-freeing latencies
>
> 2.6.15-rc1 moved the unlinking of a vma from its prio_tree and anon_vma
> into free_pgtables: so the vma is hidden from rmap and vmtruncate before
> freeing its page tables, allowing safe descent without page table lock.
> But free_pgtables is still called with preemption disabled, and Lee
> Revell has now detected high latency there.
>
> The right fix will be to rework the mmu_gathering, not to need preemption
> disabled; but for now an ugly CONFIG_PREEMPT block in free_pgtables, to
> make an initial unlinking pass with preemption enabled - made uglier by
> CONFIG_IA64 definitions (only ia64 actually uses the start and end given
> to tlb_finish_mmu, and our floor and ceiling don't quite work for those).
> These CONFIG choices being to minimize the additional TLB flushing.
>
> Signed-off-by: Hugh Dickins <[email protected]>
> Signed-off-by: Ingo Molnar <[email protected]>
> --
>
> mm/memory.c | 32 ++++++++++++++++++++++++++++++++
> 1 file changed, 32 insertions(+)
>
> Index: linux-rt-rebase.q/mm/memory.c
> ===================================================================
> --- linux-rt-rebase.q.orig/mm/memory.c
> +++ linux-rt-rebase.q/mm/memory.c
> @@ -264,18 +264,48 @@ void free_pgd_range(struct mmu_gather **
> flush_tlb_pgtables((*tlb)->mm, start, end);
> }
>
> +#ifdef CONFIG_IA64
> +#define tlb_start_addr(tlb) (tlb)->start_addr
> +#define tlb_end_addr(tlb) (tlb)->end_addr
> +#else
> +#define tlb_start_addr(tlb) 0UL /* only ia64 really uses it */
> +#define tlb_end_addr(tlb) 0UL /* only ia64 really uses it */
> +#endif
> +
> void free_pgtables(struct mmu_gather **tlb, struct vm_area_struct *vma,
> unsigned long floor, unsigned long ceiling)
> {
> +#ifdef CONFIG_PREEMPT
> + struct vm_area_struct *unlink = vma;
> + int fullmm = (*tlb)->fullmm;
> +
> + if (!vma) /* Sometimes when exiting after an oops */
> + return;
> + if (vma->vm_next)
> + tlb_finish_mmu(*tlb, tlb_start_addr(*tlb), tlb_end_addr(*tlb));
> + /*
> + * Hide vma from rmap and vmtruncate before freeeing pgtables,
> + * with preemption enabled, except when unmapping just one area.
> + */
> + while (unlink) {
> + anon_vma_unlink(unlink);
> + unlink_file_vma(unlink);
> + unlink = unlink->vm_next;
> + }
> + if (vma->vm_next)
> + *tlb = tlb_gather_mmu(vma->vm_mm, fullmm);
> +#endif
> while (vma) {
> struct vm_area_struct *next = vma->vm_next;
> unsigned long addr = vma->vm_start;
>
> +#ifndef CONFIG_PREEMPT
> /*
> * Hide vma from rmap and vmtruncate before freeing pgtables
> */
> anon_vma_unlink(vma);
> unlink_file_vma(vma);
> +#endif
>
> if (is_vm_hugetlb_page(vma)) {
> hugetlb_free_pgd_range(tlb, addr, vma->vm_end,
> @@ -288,8 +318,10 @@ void free_pgtables(struct mmu_gather **t
> && !is_vm_hugetlb_page(next)) {
> vma = next;
> next = vma->vm_next;
> +#ifndef CONFIG_PREEMPT
> anon_vma_unlink(vma);
> unlink_file_vma(vma);
> +#endif
> }
> free_pgd_range(tlb, addr, vma->vm_end,
> floor, next? next->vm_start: ceiling);
What a truly putrid patch. I am suspecting that this was a quick
get-you-out-of-trouble thing, which then got forgotten about.
We have two months to do the "right fix". Please?
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