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Subject2.6.15-rc5: latency regression vs 2.6.14 in exit_mmap->free_pgtables
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I am seeing excessive latencies (2ms+) in free_pgtables, called from
exit_mmap with 2.6.15-rc5. This is a significant regression from 2.6.14
where the maximum observed latency was less than 1ms with some tuning.
Here is the trace.

(Although the trace was captured with the -rt kernel I am only using it
for the instrumentation features - all preemption settings are the same
as mainline with PREEMPT)

preemption latency trace v1.1.5 on 2.6.15-rc5-rt4
--------------------------------------------------------------------
latency: 2267 us, #2632/2632, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0)
-----------------
| task: softirq-timer/0-3 (uid:0 nice:0 policy:1 rt_prio:1)
-----------------

_------=> CPU#
/ _-----=> irqs-off
| / _----=> need-resched
|| / _---=> hardirq/softirq
||| / _--=> preempt-depth
|||| /
||||| delay
cmd pid ||||| time | caller
\ / ||||| \ | /
evolutio-4728 0D.h2 0us : __trace_start_sched_wakeup (try_to_wake_up)
evolutio-4728 0D.h2 1us : __trace_start_sched_wakeup <<...>-3> (62 0)
evolutio-4728 0D.h. 2us : wake_up_process (wakeup_softirqd)
evolutio-4728 0D.h. 3us : scheduler_tick (update_process_times)
evolutio-4728 0D.h. 3us : sched_clock (scheduler_tick)
evolutio-4728 0D.h1 5us : task_timeslice (scheduler_tick)
evolutio-4728 0D.h. 6us : run_posix_cpu_timers (update_process_times)
evolutio-4728 0D.h1 7us : note_interrupt (__do_IRQ)
evolutio-4728 0D.h1 7us : end_8259A_irq (__do_IRQ)
evolutio-4728 0D.h1 8us : enable_8259A_irq (end_8259A_irq)
evolutio-4728 0Dnh1 10us : irq_exit (do_IRQ)
evolutio-4728 0Dn.1 11us < (608)
evolutio-4728 0.n.1 11us : unlink_file_vma (free_pgtables)
evolutio-4728 0.n.2 12us : __remove_shared_vm_struct (unlink_file_vma)
evolutio-4728 0.n.2 13us : vma_prio_tree_remove (__remove_shared_vm_struct)
evolutio-4728 0.n.2 13us : prio_tree_remove (vma_prio_tree_remove)
evolutio-4728 0.n.1 14us : preempt_schedule (unlink_file_vma)
evolutio-4728 0.n.1 15us : anon_vma_unlink (free_pgtables)
evolutio-4728 0.n.1 16us : unlink_file_vma (free_pgtables)
evolutio-4728 0.n.2 17us : __remove_shared_vm_struct (unlink_file_vma)
evolutio-4728 0.n.2 17us : vma_prio_tree_remove (__remove_shared_vm_struct)
evolutio-4728 0.n.2 18us : prio_tree_remove (vma_prio_tree_remove)
evolutio-4728 0.n.2 19us : prio_tree_replace (prio_tree_remove)
evolutio-4728 0.n.1 20us : preempt_schedule (unlink_file_vma)
evolutio-4728 0.n.1 21us : anon_vma_unlink (free_pgtables)
evolutio-4728 0.n.1 22us : preempt_schedule (anon_vma_unlink)
evolutio-4728 0.n.1 23us : kmem_cache_free (anon_vma_unlink)
evolutio-4728 0.n.1 24us : unlink_file_vma (free_pgtables)

[...]

evolutio-4728 0.n.1 2239us : unlink_file_vma (free_pgtables)
evolutio-4728 0.n.1 2240us : anon_vma_unlink (free_pgtables)
evolutio-4728 0.n.1 2241us : preempt_schedule (anon_vma_unlink)
evolutio-4728 0.n.1 2241us : kmem_cache_free (anon_vma_unlink)
evolutio-4728 0.n.1 2242us : unlink_file_vma (free_pgtables)
evolutio-4728 0.n.1 2243us : free_pgd_range (free_pgtables)
evolutio-4728 0.n.1 2243us : free_pte_range (free_pgd_range)
evolutio-4728 0.n.1 2244us : free_page_and_swap_cache (free_pte_range)
evolutio-4728 0.n.1 2245us : put_page (free_page_and_swap_cache)
evolutio-4728 0.n.1 2245us : __page_cache_release (put_page)
evolutio-4728 0.n.1 2246us : preempt_schedule (__page_cache_release)
evolutio-4728 0.n.1 2247us : free_hot_page (__page_cache_release)
evolutio-4728 0.n.1 2248us : free_hot_cold_page (free_hot_page)
evolutio-4728 0.n.1 2248us : __mod_page_state (free_hot_cold_page)
evolutio-4728 0.n.1 2250us : preempt_schedule (free_hot_cold_page)
evolutio-4728 0.n.1 2250us : __mod_page_state (free_pte_range)
evolutio-4728 0.n.. 2252us : preempt_schedule (exit_mmap)
evolutio-4728 0Dn.. 2252us : __schedule (preempt_schedule)
evolutio-4728 0Dn.. 2254us : profile_hit (__schedule)
evolutio-4728 0Dn.1 2255us+: sched_clock (__schedule)
<...>-3 0D..2 2259us+: __switch_to (__schedule)
<...>-3 0D..2 2262us : __schedule <evolutio-4728> (74 62)
<...>-3 0...1 2263us : trace_stop_sched_switched (__schedule)
<...>-3 0D..2 2264us+: trace_stop_sched_switched <<...>-3> (62 0)
<...>-3 0D..2 2266us : trace_stop_sched_switched (__schedule)

The problem is that we now do a lot more work in free_pgtables under the
mm->page_table_lock spinlock so preemption can be delayed for a long
time. Here is the change responsible:

--- ../linux-2.6.14-rt22/mm/memory.c 2005-12-05 14:47:02.000000000 -0500
+++ ../linux-2.6.15-rc5-rt2/mm/memory.c 2005-12-22 16:35:26.000000000 -0500
@@ -260,6 +261,12 @@
struct vm_area_struct *next = vma->vm_next;
unsigned long addr = vma->vm_start;

+ /*
+ * Hide vma from rmap and vmtruncate before freeing pgtables
+ */
+ anon_vma_unlink(vma);
+ unlink_file_vma(vma);
+
if (is_hugepage_only_range(vma->vm_mm, addr, HPAGE_SIZE)) {
hugetlb_free_pgd_range(tlb, addr, vma->vm_end,
floor, next? next->vm_start: ceiling);
@@ -272,6 +279,8 @@
HPAGE_SIZE)) {
vma = next;
next = vma->vm_next;
+ anon_vma_unlink(vma);
+ unlink_file_vma(vma);
}
free_pgd_range(tlb, addr, vma->vm_end,
floor, next? next->vm_start: ceiling);
What was the purpose of this change?

Lee



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