Re: [v2 PATCH] mm: mmu_gather: remove __tlb_reset_range() for force flush
From: Yang Shi
Date: Tue May 14 2019 - 13:27:36 EST
On 5/14/19 7:54 AM, Will Deacon wrote:
On Mon, May 13, 2019 at 04:01:09PM -0700, Yang Shi wrote:
On 5/13/19 9:38 AM, Will Deacon wrote:
On Fri, May 10, 2019 at 07:26:54AM +0800, Yang Shi wrote:
diff --git a/mm/mmu_gather.c b/mm/mmu_gather.c
index 99740e1..469492d 100644
--- a/mm/mmu_gather.c
+++ b/mm/mmu_gather.c
@@ -245,14 +245,39 @@ void tlb_finish_mmu(struct mmu_gather *tlb,
{
/*
* If there are parallel threads are doing PTE changes on same range
- * under non-exclusive lock(e.g., mmap_sem read-side) but defer TLB
- * flush by batching, a thread has stable TLB entry can fail to flush
- * the TLB by observing pte_none|!pte_dirty, for example so flush TLB
- * forcefully if we detect parallel PTE batching threads.
+ * under non-exclusive lock (e.g., mmap_sem read-side) but defer TLB
+ * flush by batching, one thread may end up seeing inconsistent PTEs
+ * and result in having stale TLB entries. So flush TLB forcefully
+ * if we detect parallel PTE batching threads.
+ *
+ * However, some syscalls, e.g. munmap(), may free page tables, this
+ * needs force flush everything in the given range. Otherwise this
+ * may result in having stale TLB entries for some architectures,
+ * e.g. aarch64, that could specify flush what level TLB.
*/
- if (mm_tlb_flush_nested(tlb->mm)) {
- __tlb_reset_range(tlb);
- __tlb_adjust_range(tlb, start, end - start);
+ if (mm_tlb_flush_nested(tlb->mm) && !tlb->fullmm) {
+ /*
+ * Since we can't tell what we actually should have
+ * flushed, flush everything in the given range.
+ */
+ tlb->freed_tables = 1;
+ tlb->cleared_ptes = 1;
+ tlb->cleared_pmds = 1;
+ tlb->cleared_puds = 1;
+ tlb->cleared_p4ds = 1;
+
+ /*
+ * Some architectures, e.g. ARM, that have range invalidation
+ * and care about VM_EXEC for I-Cache invalidation, need force
+ * vma_exec set.
+ */
+ tlb->vma_exec = 1;
+
+ /* Force vma_huge clear to guarantee safer flush */
+ tlb->vma_huge = 0;
+
+ tlb->start = start;
+ tlb->end = end;
}
Whilst I think this is correct, it would be interesting to see whether
or not it's actually faster than just nuking the whole mm, as I mentioned
before.
At least in terms of getting a short-term fix, I'd prefer the diff below
if it's not measurably worse.
I did a quick test with ebizzy (96 threads with 5 iterations) on my x86 VM,
it shows slightly slowdown on records/s but much more sys time spent with
fullmm flush, the below is the data.
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ nofullmmÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ fullmm
ops (records/s) ÂÂÂÂÂÂÂÂÂÂÂÂ 225606ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ 225119
sys (s)ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ 0.69ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ 1.14
It looks the slight reduction of records/s is caused by the increase of sys
time.
That's not what I expected, and I'm unable to explain why moving to fullmm
would /increase/ the system time. I would've thought the time spent doing
the invalidation would decrease, with the downside that the TLB is cold
when returning back to userspace.
FWIW, I ran 10 iterations of ebizzy on my arm64 box using a vanilla 5.1
kernel and the numbers are all over the place (see below). I think
deducing anything meaningful from this benchmark will be a challenge.
Yes, it looks so. What else benchmark do you suggest?
Will
--->8
306090 records/s
real 10.00 s
user 1227.55 s
sys 0.54 s
323547 records/s
real 10.00 s
user 1262.95 s
sys 0.82 s
409148 records/s
real 10.00 s
user 1266.54 s
sys 0.94 s
341507 records/s
real 10.00 s
user 1263.49 s
sys 0.66 s
375910 records/s
real 10.00 s
user 1259.87 s
sys 0.82 s
376152 records/s
real 10.00 s
user 1265.76 s
sys 0.96 s
358862 records/s
real 10.00 s
user 1251.13 s
sys 0.72 s
358164 records/s
real 10.00 s
user 1243.48 s
sys 0.85 s
332148 records/s
real 10.00 s
user 1260.93 s
sys 0.70 s
367021 records/s
real 10.00 s
user 1264.06 s
sys 1.43 s