| Message ID | 20210114113140.23069-1-linmiaohe@huawei.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | mm/hugetlb: avoid unnecessary hugetlb_acct_memory() call | expand |
On 14.01.21 12:31, Miaohe Lin wrote: > When gbl_reserve is 0, hugetlb_acct_memory() will do nothing except holding > and releasing hugetlb_lock. So, what's the deal then? Adding more code? If this is a performance improvement, we should spell it out. Otherwise I don't see a real benefit of this patch. > > Signed-off-by: Miaohe Lin <linmiaohe@huawei.com> > --- > mm/hugetlb.c | 3 ++- > 1 file changed, 2 insertions(+), 1 deletion(-) > > diff --git a/mm/hugetlb.c b/mm/hugetlb.c > index 737b2dce19e6..fe2da9ad6233 100644 > --- a/mm/hugetlb.c > +++ b/mm/hugetlb.c > @@ -5241,7 +5241,8 @@ long hugetlb_unreserve_pages(struct inode *inode, long start, long end, > * reservations to be released may be adjusted. > */ > gbl_reserve = hugepage_subpool_put_pages(spool, (chg - freed)); > - hugetlb_acct_memory(h, -gbl_reserve); > + if (gbl_reserve) > + hugetlb_acct_memory(h, -gbl_reserve); > > return 0; > } >
On 1/14/21 4:32 AM, David Hildenbrand wrote: > On 14.01.21 12:31, Miaohe Lin wrote: >> When gbl_reserve is 0, hugetlb_acct_memory() will do nothing except holding >> and releasing hugetlb_lock. > > So, what's the deal then? Adding more code? > > If this is a performance improvement, we should spell it out. Otherwise > I don't see a real benefit of this patch. > Thanks for finding/noticing this. As David points out, the commit message should state that this is a performance improvement. Mention that such a change avoids an unnecessary hugetlb_lock lock/unlock cycle. You can also mention that this unnecessary lock cycle is happening on 'most' hugetlb munmap operations. >> >> Signed-off-by: Miaohe Lin <linmiaohe@huawei.com> >> --- >> mm/hugetlb.c | 3 ++- >> 1 file changed, 2 insertions(+), 1 deletion(-) >> >> diff --git a/mm/hugetlb.c b/mm/hugetlb.c >> index 737b2dce19e6..fe2da9ad6233 100644 >> --- a/mm/hugetlb.c >> +++ b/mm/hugetlb.c >> @@ -5241,7 +5241,8 @@ long hugetlb_unreserve_pages(struct inode *inode, long start, long end, >> * reservations to be released may be adjusted. >> */ >> gbl_reserve = hugepage_subpool_put_pages(spool, (chg - freed)); >> - hugetlb_acct_memory(h, -gbl_reserve); >> + if (gbl_reserve) >> + hugetlb_acct_memory(h, -gbl_reserve); It is true that gbl_reserve is likely to be 0 in this code path. However, there are other code paths where hugetlb_acct_memory is called with a delta value of 0 as well. I would rather see a simple check at the beginning of hugetlb_acct_memory like. if (!delta) return 0;
Hi: On 2021/1/15 3:16, Mike Kravetz wrote: > On 1/14/21 4:32 AM, David Hildenbrand wrote: >> On 14.01.21 12:31, Miaohe Lin wrote: >>> When gbl_reserve is 0, hugetlb_acct_memory() will do nothing except holding >>> and releasing hugetlb_lock. >> >> So, what's the deal then? Adding more code? >> >> If this is a performance improvement, we should spell it out. Otherwise >> I don't see a real benefit of this patch. >> > > Thanks for finding/noticing this. > > As David points out, the commit message should state that this is a > performance improvement. Mention that such a change avoids an unnecessary > hugetlb_lock lock/unlock cycle. You can also mention that this unnecessary > lock cycle is happening on 'most' hugetlb munmap operations. > My bad. I should spell this out explicitly. Many thanks for both of you. >>> >>> Signed-off-by: Miaohe Lin <linmiaohe@huawei.com> >>> --- >>> mm/hugetlb.c | 3 ++- >>> 1 file changed, 2 insertions(+), 1 deletion(-) >>> >>> diff --git a/mm/hugetlb.c b/mm/hugetlb.c >>> index 737b2dce19e6..fe2da9ad6233 100644 >>> --- a/mm/hugetlb.c >>> +++ b/mm/hugetlb.c >>> @@ -5241,7 +5241,8 @@ long hugetlb_unreserve_pages(struct inode *inode, long start, long end, >>> * reservations to be released may be adjusted. >>> */ >>> gbl_reserve = hugepage_subpool_put_pages(spool, (chg - freed)); >>> - hugetlb_acct_memory(h, -gbl_reserve); >>> + if (gbl_reserve) >>> + hugetlb_acct_memory(h, -gbl_reserve); > > It is true that gbl_reserve is likely to be 0 in this code path. However, > there are other code paths where hugetlb_acct_memory is called with a delta > value of 0 as well. I would rather see a simple check at the beginning of > hugetlb_acct_memory like. > > if (!delta) > return 0; > Sounds good. Will do it in v2. Many thanks again.
On 15.01.21 03:04, Miaohe Lin wrote: > Hi: > > On 2021/1/15 3:16, Mike Kravetz wrote: >> On 1/14/21 4:32 AM, David Hildenbrand wrote: >>> On 14.01.21 12:31, Miaohe Lin wrote: >>>> When gbl_reserve is 0, hugetlb_acct_memory() will do nothing except holding >>>> and releasing hugetlb_lock. >>> >>> So, what's the deal then? Adding more code? >>> >>> If this is a performance improvement, we should spell it out. Otherwise >>> I don't see a real benefit of this patch. >>> >> >> Thanks for finding/noticing this. >> >> As David points out, the commit message should state that this is a >> performance improvement. Mention that such a change avoids an unnecessary >> hugetlb_lock lock/unlock cycle. You can also mention that this unnecessary >> lock cycle is happening on 'most' hugetlb munmap operations. >> > > My bad. I should spell this out explicitly. Many thanks for both of you. With the "lock cycle is happening on 'most' hugetlb munmap operations" part added Reviewed-by: David Hildenbrand <david@redhat.com> Thanks!
diff --git a/mm/hugetlb.c b/mm/hugetlb.c index 737b2dce19e6..fe2da9ad6233 100644 --- a/mm/hugetlb.c +++ b/mm/hugetlb.c @@ -5241,7 +5241,8 @@ long hugetlb_unreserve_pages(struct inode *inode, long start, long end, * reservations to be released may be adjusted. */ gbl_reserve = hugepage_subpool_put_pages(spool, (chg - freed)); - hugetlb_acct_memory(h, -gbl_reserve); + if (gbl_reserve) + hugetlb_acct_memory(h, -gbl_reserve); return 0; }
When gbl_reserve is 0, hugetlb_acct_memory() will do nothing except holding and releasing hugetlb_lock. Signed-off-by: Miaohe Lin <linmiaohe@huawei.com> --- mm/hugetlb.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-)