From: Xing Zhengjun <zhengjun.xing@linux.intel.com>
To: Linus Torvalds <torvalds@linux-foundation.org>,
kernel test robot <oliver.sang@intel.com>
Cc: Jason Gunthorpe <jgg@nvidia.com>,
John Hubbard <jhubbard@nvidia.com>, Jan Kara <jack@suse.cz>,
Peter Xu <peterx@redhat.com>,
Andrea Arcangeli <aarcange@redhat.com>,
"Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>,
Christoph Hellwig <hch@lst.de>, Hugh Dickins <hughd@google.com>,
Jann Horn <jannh@google.com>,
Kirill Shutemov <kirill@shutemov.name>,
Kirill Tkhai <ktkhai@virtuozzo.com>,
Leon Romanovsky <leonro@nvidia.com>,
Michal Hocko <mhocko@suse.com>, Oleg Nesterov <oleg@redhat.com>,
Andrew Morton <akpm@linux-foundation.org>,
LKML <linux-kernel@vger.kernel.org>,
lkp@lists.01.org, kernel test robot <lkp@intel.com>
Subject: Re: [LKP] Re: [mm/gup] 57efa1fe59: will-it-scale.per_thread_ops -9.2% regression
Date: Fri, 4 Jun 2021 16:37:53 +0800 [thread overview]
Message-ID: <2a1977e1-4fd1-065a-214f-51c6b40cc3d2@linux.intel.com> (raw)
In-Reply-To: <CAHk-=whTEC_GVYu=WfvUagNvHdoTALEDg8uqK3V6aMDwg2KMRA@mail.gmail.com>
Hi Linus,
On 5/25/2021 11:11 AM, Linus Torvalds wrote:
> On Mon, May 24, 2021 at 5:00 PM kernel test robot <oliver.sang@intel.com> wrote:
>> FYI, we noticed a -9.2% regression of will-it-scale.per_thread_ops due to commit:
>> commit: 57efa1fe5957694fa541c9062de0a127f0b9acb0 ("mm/gup: prevent gup_fast from racing with COW during fork")
> Hmm. This looks like one of those "random fluctuations" things.
>
> It would be good to hear if other test-cases also bisect to the same
> thing, but this report already says:
>
>> In addition to that, the commit also has significant impact on the following tests:
>>
>> +------------------+---------------------------------------------------------------------------------+
>> | testcase: change | will-it-scale: will-it-scale.per_thread_ops 3.7% improvement |
> which does kind of reinforce that "this benchmark gives unstable numbers".
>
> The perf data doesn't even mention any of the GUP paths, and on the
> pure fork path the biggest impact would be:
>
> (a) maybe "struct mm_struct" changed in size or had a different cache layout
I move "write_protect_seq" to the tail of the "struct mm_struct", the
regression reduced to -3.6%. The regression should relate to the cache
layout.
=========================================================================================
tbox_group/testcase/rootfs/kconfig/compiler/nr_task/mode/test/cpufreq_governor/ucode:
lkp-icl-2sp1/will-it-scale/debian-10.4-x86_64-20200603.cgz/x86_64-rhel-8.3/gcc-9/50%/thread/mmap1/performance/0xb000280
commit:
c28b1fc70390df32e29991eedd52bd86e7aba080
57efa1fe5957694fa541c9062de0a127f0b9acb0
f6a9c27882d51ff551e15522992d3725c342372d (the test patch)
c28b1fc70390df32 57efa1fe5957694fa541c9062de f6a9c27882d51ff551e15522992
---------------- --------------------------- ---------------------------
%stddev %change %stddev %change %stddev
\ | \ | \
341938 -9.0% 311218 ± 2% -3.6% 329513
will-it-scale.48.threads
7123 -9.0% 6483 ± 2% -3.6% 6864
will-it-scale.per_thread_ops
341938 -9.0% 311218 ± 2% -3.6% 329513
will-it-scale.workload
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 915f4f100383..34bb2a01806c 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -447,13 +447,6 @@ struct mm_struct {
*/
atomic_t has_pinned;
- /**
- * @write_protect_seq: Locked when any thread is write
- * protecting pages mapped by this mm to enforce a later
COW,
- * for instance during page table copying for fork().
- */
- seqcount_t write_protect_seq;
-
#ifdef CONFIG_MMU
atomic_long_t pgtables_bytes; /* PTE page table pages */
#endif
@@ -564,6 +557,12 @@ struct mm_struct {
#ifdef CONFIG_IOMMU_SUPPORT
u32 pasid;
#endif
+ /**
+ * @write_protect_seq: Locked when any thread is write
+ * protecting pages mapped by this mm to enforce a
later COW,
+ * for instance during page table copying for fork().
+ */
+ seqcount_t write_protect_seq;
} __randomize_layout;
/*
>
> (b) two added (nonatomic) increment operations in the fork path due
> to the seqcount
>
> and I'm not seeing what would cause that 9% change. Obviously cache
> placement has done it before.
>
> If somebody else sees something that I'm missing, please holler. But
> I'll ignore this as "noise" otherwise.
>
> Linus
> _______________________________________________
> LKP mailing list -- lkp@lists.01.org
> To unsubscribe send an email to lkp-leave@lists.01.org
--
Zhengjun Xing
prev parent reply other threads:[~2021-06-04 8:38 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-05-25 3:16 [mm/gup] 57efa1fe59: will-it-scale.per_thread_ops -9.2% regression kernel test robot
2021-05-25 3:11 ` Linus Torvalds
2021-06-04 7:04 ` Feng Tang
2021-06-04 7:52 ` Feng Tang
2021-06-04 17:57 ` Linus Torvalds
2021-06-06 10:16 ` Feng Tang
2021-06-06 19:20 ` Linus Torvalds
2021-06-06 22:13 ` Waiman Long
2021-06-07 6:05 ` Feng Tang
2021-06-08 0:03 ` Linus Torvalds
2021-06-04 17:58 ` John Hubbard
2021-06-06 4:47 ` Feng Tang
2021-06-04 8:37 ` Xing Zhengjun [this message]
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