Messages in this thread Patch in this message |  | | Date | Thu, 29 Aug 2002 13:30:04 -0700 | From | Andrew Morton <> | Subject | Re: [PATCH] low-latency zap_page_range() |
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Robert Love wrote: > > Andrew, > > Attached patch implements a low latency version of "zap_page_range()". >
This doesn't quite do the right thing on SMP.
Note that pages which are to be torn down are buffered in the mmu_gather_t array. The kernel throws away 507 pages at a time - this is to reduce the frequency of global TLB invalidations. (The 507 is, I assume, designed to make the mmu_gather_t be 2048 bytes in size. I recently broke that math, and need to fix it up).
However with your change, we'll only ever put 256 pages into the mmu_gather_t. Half of that thing's buffer is unused and the invalidation rate will be doubled during teardown of large address ranges.
I suggest that you make ZAP_BLOCK_SIZE be equal to FREE_PTE_NR on SMP, and 256 on UP.
(We could get fancier and do something like:
tlb = tlb_gather_mmu(mm, 0): while (size) { ... unmap_page_range(ZAP_BLOCK_SIZE pages); tlb_flush_mmu(...); cond_resched_lock(); } tlb_finish_mmu(..); spin_unlock(page_table_lock);
but I don't think that passes the benefit-versus-complexity test.)
Also, if the kernel is not compiled for preemption then we're doing a little bit of extra work to no advantage, yes? We can avoid doing that by setting ZAP_BLOCK_SIZE to infinity.
How does this altered version look? All I changed was the ZAP_BLOCK_SIZE initialisation.
--- 2.5.32/include/linux/sched.h~llzpr Thu Aug 29 13:01:01 2002 +++ 2.5.32-akpm/include/linux/sched.h Thu Aug 29 13:01:01 2002 @@ -907,6 +907,34 @@ static inline void cond_resched(void) __cond_resched(); } +#ifdef CONFIG_PREEMPT + +/* + * cond_resched_lock() - if a reschedule is pending, drop the given lock, + * call schedule, and on return reacquire the lock. + * + * Note: this does not assume the given lock is the _only_ lock held. + * The kernel preemption counter gives us "free" checking that we are + * atomic -- let's use it. + */ +static inline void cond_resched_lock(spinlock_t * lock) +{ + if (need_resched() && preempt_count() == 1) { + _raw_spin_unlock(lock); + preempt_enable_no_resched(); + __cond_resched(); + spin_lock(lock); + } +} + +#else + +static inline void cond_resched_lock(spinlock_t * lock) +{ +} + +#endif + /* Reevaluate whether the task has signals pending delivery. This is required every time the blocked sigset_t changes. Athread cathreaders should have t->sigmask_lock. */ --- 2.5.32/mm/memory.c~llzpr Thu Aug 29 13:01:01 2002 +++ 2.5.32-akpm/mm/memory.c Thu Aug 29 13:26:21 2002 @@ -389,8 +389,8 @@ void unmap_page_range(mmu_gather_t *tlb, { pgd_t * dir; - if (address >= end) - BUG(); + BUG_ON(address >= end); + dir = pgd_offset(vma->vm_mm, address); tlb_start_vma(tlb, vma); do { @@ -401,30 +401,53 @@ void unmap_page_range(mmu_gather_t *tlb, tlb_end_vma(tlb, vma); } -/* - * remove user pages in a given range. +#if defined(CONFIG_SMP) && defined(CONFIG_PREEMPT) +#define ZAP_BLOCK_SIZE (FREE_PTE_NR * PAGE_SIZE) +#endif + +#if !defined(CONFIG_SMP) && defined(CONFIG_PREEMPT) +#define ZAP_BLOCK_SIZE (256 * PAGE_SIZE) +#endif + +#if !defined(CONFIG_PREEMPT) +#define ZAP_BLOCK_SIZE (~(0UL)) +#endif + +/** + * zap_page_range - remove user pages in a given range + * @vma: vm_area_struct holding the applicable pages + * @address: starting address of pages to zap + * @size: number of bytes to zap */ void zap_page_range(struct vm_area_struct *vma, unsigned long address, unsigned long size) { struct mm_struct *mm = vma->vm_mm; mmu_gather_t *tlb; - unsigned long start = address, end = address + size; + unsigned long end, block; - /* - * This is a long-lived spinlock. That's fine. - * There's no contention, because the page table - * lock only protects against kswapd anyway, and - * even if kswapd happened to be looking at this - * process we _want_ it to get stuck. - */ - if (address >= end) - BUG(); spin_lock(&mm->page_table_lock); - flush_cache_range(vma, address, end); - tlb = tlb_gather_mmu(mm, 0); - unmap_page_range(tlb, vma, address, end); - tlb_finish_mmu(tlb, start, end); + /* + * This was once a long-held spinlock. Now we break the + * work up into ZAP_BLOCK_SIZE units and relinquish the + * lock after each interation. This drastically lowers + * lock contention and allows for a preemption point. + */ + while (size) { + block = (size > ZAP_BLOCK_SIZE) ? ZAP_BLOCK_SIZE : size; + end = address + block; + + flush_cache_range(vma, address, end); + tlb = tlb_gather_mmu(mm, 0); + unmap_page_range(tlb, vma, address, end); + tlb_finish_mmu(tlb, address, end); + + cond_resched_lock(&mm->page_table_lock); + + address += block; + size -= block; + } + spin_unlock(&mm->page_table_lock); } . - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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