Messages in this thread Patch in this message | | | From | Amir Goldstein <> | Date | Sun, 30 Sep 2018 12:16:45 +0300 | Subject | Re: [LKP] [fsnotify] 60f7ed8c7c: will-it-scale.per_thread_ops -5.9% regression |
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On Sun, Sep 30, 2018 at 12:00 PM Amir Goldstein <amir73il@gmail.com> wrote: > > On Sun, Sep 30, 2018 at 9:50 AM kernel test robot <rong.a.chen@intel.com> wrote: > > > > Greeting, > > > > FYI, we noticed a -5.9% regression of will-it-scale.per_thread_ops due to commit: > > > > > > commit: 60f7ed8c7c4d06aeda448c6da74621552ee739aa ("fsnotify: send path type events to group with super block marks") > > https://git.kernel.org/cgit/linux/kernel/git/next/linux-next.git master > > > > in testcase: will-it-scale > > on test machine: 88 threads Intel(R) Xeon(R) CPU E5-2699 v4 @ 2.20GHz with 64G memory > > with following parameters: > > > > nr_task: 16 > > mode: thread > > test: unlink2 > > cpufreq_governor: performance > > > > test-description: Will It Scale takes a testcase and runs it from 1 through to n parallel copies to see if the testcase will scale. It builds both a process and threads based test in order to see any differences between the two. > > test-url: https://github.com/antonblanchard/will-it-scale > > > > > > > > Details are as below: > > --------------------------------------------------------------------------------------------------> > > > > > > To reproduce: > > > > git clone https://github.com/intel/lkp-tests.git > > cd lkp-tests > > bin/lkp install job.yaml # job file is attached in this email > > bin/lkp run job.yaml > > > > ========================================================================================= > > compiler/cpufreq_governor/kconfig/mode/nr_task/rootfs/tbox_group/test/testcase: > > gcc-7/performance/x86_64-rhel-7.2/thread/16/debian-x86_64-2018-04-03.cgz/lkp-bdw-ep3d/unlink2/will-it-scale > > > > commit: > > 1e6cb72399 ("fsnotify: add super block object type") > > 60f7ed8c7c ("fsnotify: send path type events to group with super block marks") > > > > I have to admit this looks strange. > All this commit does is dereference mnt->mnt.mnt_sb and then > sb->s_fsnotify_mask/sb->s_fsnotify_marks to find that they are zero. > AFAICT there should be no extra contention added by this commit and it's > hard to believe that parallel unlink workload would suffer from this change. > > I will try to install lkp-tests to verify this on my own system, but > until proven > otherwise I will regard this as false positive. >
Actually, is it possible to ask for a private test with the following patch to optimize out an unneeded srcu_derefence(). This optimization (assuming it is correct) could in fact improve scaling compared to upstream, because there are already 2 calls to fsnotify_first_mark in the code. The blamed commit just adds a 3rd one. I am assuming that() in the test there is an fsnotify mount mark (maybe setup by systemd) otherwise, the optimization in line 351 would have not reached the extra fsnotify_first_mark() call. Can you confirm or disprove the assumption that an fanotify mount mark is present during the test?
Thanks, Amir.
---- diff --git a/fs/notify/fsnotify.c b/fs/notify/fsnotify.c index 422fbc6dffde..8d45d82e09ff 100644 --- a/fs/notify/fsnotify.c +++ b/fs/notify/fsnotify.c @@ -246,6 +246,9 @@ static struct fsnotify_mark *fsnotify_first_mark(struct fsnotify_mark_connector struct fsnotify_mark_connector *conn; struct hlist_node *node = NULL;
+ if (!*connp) + return NULL; + conn = srcu_dereference(*connp, &fsnotify_mark_srcu); if (conn) node = srcu_dereference(conn->list.first, &fsnotify_mark_srcu);
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