Messages in this thread Patch in this message | | | Subject | [PATCH] updated low-latency zap_page_range | From | Robert Love <> | Date | 24 Jul 2002 17:29:35 -0700 |
| |
Andrew and Linus,
The lock hold time in zap_page_range is horrid. This patch breaks the work up into chunks and relinquishes the lock after each iteration. This drastically lowers latency by creating a preemption point, as well as lowering lock contention.
This patch is updated over the previous: per Linus's suggestion, we now call a new "cond_resched_lock()" function. Per Andrew's suggestion, it checks the preempt_count as to not allow an improper preemption. However, we can now longer allow the conditional reschedule without CONFIG_PREEMPT here (since there is no way to know if it is safe) so it becomes a nop.
This lowers the maximum latency in zap_page_range from 10~20ms (on a dual Athlon - one of the worst latencies recorded) to unmeasurable.
I made a couple other cleanups and optimizations:
- remove unneeded dir variable and call to pgd_offset - nothing uses this anymore as the code was pushed to unmap_page_range
- removed duplicated start variable - it is the same as address
- BUG -> BUG_ON in unmap_page_range
- remove redundant BUG from zap_page_range - the same check is done in unmap_page_range
- better comments
Patch is against 2.5.28, please apply.
Robert Love
diff -urN linux-2.5.28/include/linux/sched.h linux/include/linux/sched.h --- linux-2.5.28/include/linux/sched.h Wed Jul 24 14:03:20 2002 +++ linux/include/linux/sched.h Wed Jul 24 17:21:29 2002 @@ -888,6 +888,24 @@ __cond_resched(); } +/* + * cond_resched_lock() - if a reschedule is pending, drop the given lock, + * call schedule, and on return reacquire the lock. + * + * Note: this assumes the given lock is the _only_ held lock and otherwise + * you are not atomic. The kernel preemption counter gives us "free" + * checking that this is really the only lock held -- 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); + } +} + /* 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. */ diff -urN linux-2.5.28/mm/memory.c linux/mm/memory.c --- linux-2.5.28/mm/memory.c Wed Jul 24 14:03:27 2002 +++ linux/mm/memory.c Wed Jul 24 17:20:58 2002 @@ -390,8 +390,8 @@ { pgd_t * dir; - if (address >= end) - BUG(); + BUG_ON(address >= end); + dir = pgd_offset(vma->vm_mm, address); tlb_start_vma(tlb, vma); do { @@ -402,33 +402,43 @@ tlb_end_vma(tlb, vma); } -/* - * remove user pages in a given range. +#define ZAP_BLOCK_SIZE (256 * PAGE_SIZE) /* how big a chunk we loop over */ + +/** + * 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; - pgd_t * dir; - unsigned long start = address, end = address + size; + unsigned long end, block; - dir = pgd_offset(mm, address); + spin_lock(&mm->page_table_lock); /* - * 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. + * 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. */ - if (address >= end) - BUG(); - spin_lock(&mm->page_table_lock); - flush_cache_range(vma, address, end); + 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; + } - tlb = tlb_gather_mmu(mm, 0); - unmap_page_range(tlb, vma, address, end); - tlb_finish_mmu(tlb, start, end); 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/
| |