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Message-ID: <1f3a6d05-2756-93fd-a380-df808c94ece8@intel.com>
Date: Wed, 20 Dec 2017 18:24:24 +0800
From: kemi <kemi.wang@...el.com>
To: Michal Hocko <mhocko@...nel.org>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
Andrew Morton <akpm@...ux-foundation.org>,
Vlastimil Babka <vbabka@...e.cz>,
Mel Gorman <mgorman@...hsingularity.net>,
Johannes Weiner <hannes@...xchg.org>,
Christopher Lameter <cl@...ux.com>,
YASUAKI ISHIMATSU <yasu.isimatu@...il.com>,
Andrey Ryabinin <aryabinin@...tuozzo.com>,
Nikolay Borisov <nborisov@...e.com>,
Pavel Tatashin <pasha.tatashin@...cle.com>,
David Rientjes <rientjes@...gle.com>,
Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
Dave <dave.hansen@...ux.intel.com>,
Andi Kleen <andi.kleen@...el.com>,
Tim Chen <tim.c.chen@...el.com>,
Jesper Dangaard Brouer <brouer@...hat.com>,
Ying Huang <ying.huang@...el.com>,
Aaron Lu <aaron.lu@...el.com>, Aubrey Li <aubrey.li@...el.com>,
Linux MM <linux-mm@...ck.org>,
Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 4/5] mm: use node_page_state_snapshot to avoid
deviation
On 2017年12月20日 18:06, Michal Hocko wrote:
> 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.
>
That is because we have much larger threshold size for NUMA counters, that means larger
deviation. So, the number in local cpus may not be simply ignored.
>>>> Signed-off-by: Kemi Wang <kemi.wang@...el.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
>>>>
>>>
>
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