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SubjectRe: [PATCH v2 4/5] mm: use node_page_state_snapshot to avoid deviation
On Wed 20-12-17 14:07:35, kemi wrote:
>
>
> On 2017年12月19日 20:43, Michal Hocko wrote:
> > On Tue 19-12-17 14:39:25, Kemi Wang wrote:
> >> To avoid deviation, this patch uses node_page_state_snapshot instead of
> >> node_page_state for node page stats query.
> >> e.g. cat /proc/zoneinfo
> >> cat /sys/devices/system/node/node*/vmstat
> >> cat /sys/devices/system/node/node*/numastat
> >>
> >> As it is a slow path and would not be read frequently, I would worry about
> >> it.
> >
> > The changelog doesn't explain why these counters needs any special
> > treatment. _snapshot variants where used only for internal handling
> > where the precision really mattered. We do not have any in-tree user and
> > Jack has removed this by http://lkml.kernel.org/r/20171122094416.26019-1-jack@suse.cz
> > which is already sitting in the mmotm tree. We can re-add it but that
> > would really require a _very good_ reason.
> >
>
> Assume we have *nr* cpus, and threshold size is *t*. Thus, the maximum deviation is nr*t.
> Currently, Skylake platform has hundreds of CPUs numbers and the number is still
> increasing. Also, even the threshold size is kept to 125 at maximum (32765
> for NUMA counters now), the deviation is just a little too big as I have mentioned in
> the log. I tend to sum the number in local cpus up when query the global stats.

This is a general problem of pcp accounting. So if it needs to be
addressed then do it the same way for all stats.

> Also, node_page_state_snapshot is only called in slow path and I don't think that
> would be a big problem.
>
> Anyway, it is a matter of taste. I just think it's better to have.

You are making numastats special and I yet haven't heard any sounds
arguments for that. But that should be discussed in the respective
patch.

> >> Signed-off-by: Kemi Wang <kemi.wang@intel.com>
> >> ---
> >> drivers/base/node.c | 17 ++++++++++-------
> >> mm/vmstat.c | 2 +-
> >> 2 files changed, 11 insertions(+), 8 deletions(-)
> >>
> >> diff --git a/drivers/base/node.c b/drivers/base/node.c
> >> index a045ea1..cf303f8 100644
> >> --- a/drivers/base/node.c
> >> +++ b/drivers/base/node.c
> >> @@ -169,12 +169,15 @@ static ssize_t node_read_numastat(struct device *dev,
> >> "interleave_hit %lu\n"
> >> "local_node %lu\n"
> >> "other_node %lu\n",
> >> - node_page_state(NODE_DATA(dev->id), NUMA_HIT),
> >> - node_page_state(NODE_DATA(dev->id), NUMA_MISS),
> >> - node_page_state(NODE_DATA(dev->id), NUMA_FOREIGN),
> >> - node_page_state(NODE_DATA(dev->id), NUMA_INTERLEAVE_HIT),
> >> - node_page_state(NODE_DATA(dev->id), NUMA_LOCAL),
> >> - node_page_state(NODE_DATA(dev->id), NUMA_OTHER));
> >> + node_page_state_snapshot(NODE_DATA(dev->id), NUMA_HIT),
> >> + node_page_state_snapshot(NODE_DATA(dev->id), NUMA_MISS),
> >> + node_page_state_snapshot(NODE_DATA(dev->id),
> >> + NUMA_FOREIGN),
> >> + node_page_state_snapshot(NODE_DATA(dev->id),
> >> + NUMA_INTERLEAVE_HIT),
> >> + node_page_state_snapshot(NODE_DATA(dev->id), NUMA_LOCAL),
> >> + node_page_state_snapshot(NODE_DATA(dev->id),
> >> + NUMA_OTHER));
> >> }
> >>
> >> static DEVICE_ATTR(numastat, S_IRUGO, node_read_numastat, NULL);
> >> @@ -194,7 +197,7 @@ static ssize_t node_read_vmstat(struct device *dev,
> >> for (i = 0; i < NR_VM_NODE_STAT_ITEMS; i++)
> >> n += sprintf(buf+n, "%s %lu\n",
> >> vmstat_text[i + NR_VM_ZONE_STAT_ITEMS],
> >> - node_page_state(pgdat, i));
> >> + node_page_state_snapshot(pgdat, i));
> >>
> >> return n;
> >> }
> >> diff --git a/mm/vmstat.c b/mm/vmstat.c
> >> index 64e08ae..d65f28d 100644
> >> --- a/mm/vmstat.c
> >> +++ b/mm/vmstat.c
> >> @@ -1466,7 +1466,7 @@ static void zoneinfo_show_print(struct seq_file *m, pg_data_t *pgdat,
> >> for (i = 0; i < NR_VM_NODE_STAT_ITEMS; i++) {
> >> seq_printf(m, "\n %-12s %lu",
> >> vmstat_text[i + NR_VM_ZONE_STAT_ITEMS],
> >> - node_page_state(pgdat, i));
> >> + node_page_state_snapshot(pgdat, i));
> >> }
> >> }
> >> seq_printf(m,
> >> --
> >> 2.7.4
> >>
> >

--
Michal Hocko
SUSE Labs

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