Re: [PATCH v2 4/4] zram: report maximum used memory

From: David Horner
Date: Wed Aug 20 2014 - 03:38:35 EST

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On Wed, Aug 20, 2014 at 2:53 AM, Minchan Kim <minchan@xxxxxxxxxx> wrote:
> On Wed, Aug 20, 2014 at 02:26:50AM -0400, David Horner wrote:
>> On Tue, Aug 19, 2014 at 3:54 AM, Minchan Kim <minchan@xxxxxxxxxx> wrote:
>> > Normally, zram user could get maximum memory usage zram consumed
>> > via polling mem_used_total with sysfs in userspace.
>> >
>> > But it has a critical problem because user can miss peak memory
>> > usage during update inverval of polling. For avoiding that,
>> > user should poll it with shorter interval(ie, 0.0000000001s)
>> > with mlocking to avoid page fault delay when memory pressure
>> > is heavy. It would be troublesome.
>> >
>> > This patch adds new knob "mem_used_max" so user could see
>> > the maximum memory usage easily via reading the knob and reset
>> > it via "echo 0 > /sys/block/zram0/mem_used_max".
>> >
>> > Signed-off-by: Minchan Kim <minchan@xxxxxxxxxx>
>> > ---
>> > Documentation/ABI/testing/sysfs-block-zram | 10 +++++
>> > Documentation/blockdev/zram.txt | 1 +
>> > drivers/block/zram/zram_drv.c | 60 +++++++++++++++++++++++++++++-
>> > drivers/block/zram/zram_drv.h | 1 +
>> > 4 files changed, 70 insertions(+), 2 deletions(-)
>> >
>> > diff --git a/Documentation/ABI/testing/sysfs-block-zram b/Documentation/ABI/testing/sysfs-block-zram
>> > index 025331c19045..ffd1ea7443dd 100644
>> > --- a/Documentation/ABI/testing/sysfs-block-zram
>> > +++ b/Documentation/ABI/testing/sysfs-block-zram
>> > @@ -120,6 +120,16 @@ Description:
>> > statistic.
>> > Unit: bytes
>> >
>> > +What: /sys/block/zram<id>/mem_used_max
>> > +Date: August 2014
>> > +Contact: Minchan Kim <minchan@xxxxxxxxxx>
>> > +Description:
>> > + The mem_used_max file is read/write and specifies the amount
>> > + of maximum memory zram have consumed to store compressed data.
>> > + For resetting the value, you should do "echo 0". Otherwise,
>> > + you could see -EINVAL.
>> > + Unit: bytes
>> > +
>> > What: /sys/block/zram<id>/mem_limit
>> > Date: August 2014
>> > Contact: Minchan Kim <minchan@xxxxxxxxxx>
>> > diff --git a/Documentation/blockdev/zram.txt b/Documentation/blockdev/zram.txt
>> > index 9f239ff8c444..3b2247c2d4cf 100644
>> > --- a/Documentation/blockdev/zram.txt
>> > +++ b/Documentation/blockdev/zram.txt
>> > @@ -107,6 +107,7 @@ size of the disk when not in use so a huge zram is wasteful.
>> > orig_data_size
>> > compr_data_size
>> > mem_used_total
>> > + mem_used_max
>> >
>> > 8) Deactivate:
>> > swapoff /dev/zram0
>> > diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
>> > index adc91c7ecaef..e4d44842a91d 100644
>> > --- a/drivers/block/zram/zram_drv.c
>> > +++ b/drivers/block/zram/zram_drv.c
>> > @@ -149,6 +149,40 @@ static ssize_t mem_limit_store(struct device *dev,
>> > return len;
>> > }
>> >
>> > +static ssize_t mem_used_max_show(struct device *dev,
>> > + struct device_attribute *attr, char *buf)
>> > +{
>> > + u64 val = 0;
>> > + struct zram *zram = dev_to_zram(dev);
>> > +
>> > + down_read(&zram->init_lock);
>> > + if (init_done(zram))
>> > + val = atomic64_read(&zram->stats.max_used_pages);
>> > + up_read(&zram->init_lock);
>> > +
>> > + return scnprintf(buf, PAGE_SIZE, "%llu\n", val << PAGE_SHIFT);
>> > +}
>> > +
>> > +static ssize_t mem_used_max_store(struct device *dev,
>> > + struct device_attribute *attr, const char *buf, size_t len)
>> > +{
>> > + u64 limit;
>> > + struct zram *zram = dev_to_zram(dev);
>> > + struct zram_meta *meta = zram->meta;
>> > +
>> > - limit = memparse(buf, NULL);
>> > - if (0 != limit)
>>
>> we wanted explicit "0" and nothing else for extensibility
>>
>> if (len != 1 || *buf != "0")
>>
>
> I wanted to work with "0", "0K", "0M", "0G" but agree it's meaningless
> at the moment so your version is better.
>
>
>> > + return -EINVAL;
>> > +
>> > + down_read(&zram->init_lock);
>> > + if (init_done(zram))
>> > + atomic64_set(&zram->stats.max_used_pages,
>> > + zs_get_total_size(meta->mem_pool));
>> > + up_read(&zram->init_lock);
>> > +
>> > + return len;
>> return 1;
>>
>> the standard convention is to return used amount of buffer
>
> If I follow your suggestion, len should be 1 right before returning
> so no problem for functionality POV but I agree explicit "1" is better
> for readability so your version is better, better.
>
>>
>>
>>
>> > +}
>> > +
>> > static ssize_t max_comp_streams_store(struct device *dev,
>> > struct device_attribute *attr, const char *buf, size_t len)
>> > {
>> > @@ -461,6 +495,26 @@ out_cleanup:
>> > return ret;
>> > }
>> >
>> > +static bool check_limit(struct zram *zram)
>> > +{
>> > + unsigned long alloced_pages;
>> > + u64 old_max, cur_max;
>> > + struct zram_meta *meta = zram->meta;
>> > +
>> > + do {
>> > + alloced_pages = zs_get_total_size(meta->mem_pool);
>> > + if (zram->limit_pages && alloced_pages > zram->limit_pages)
>> > + return false;
>> > +
>> > + old_max = cur_max = atomic64_read(&zram->stats.max_used_pages);
>> > + if (alloced_pages > cur_max)
>> > + old_max = atomic64_cmpxchg(&zram->stats.max_used_pages,
>> > + cur_max, alloced_pages);
>> > + } while (old_max != cur_max);
>> > +
>> > + return true;
>> > +}
>> > +
>>
>> Check_limit does more than check limit - it has a substantial side
>> effect of updating max used.
>
> Hmm, Normally, limit check is best place to update the max although
> function name imply just checking the limit and I don't think
> code piece for max updating doesn't hurt readbilty.
> If you or other reviewer is strong against, I will be happy to
> factor out part of max updating into another function because
> I think it's just preference problem for small logic and don't want
> to waste argue for that.
>
> If you really want it, pz, ping me again.
>
>>
>> Basically if we already allocated the buffer and our alloced_pages is
>> less than the limit then we are good to go.
>
> Yeb.
>
>>
>> It is the race to update that we need to have the cmpxchg.
>> And maybe a helper function would aid readability - not sure, see next point.
>>
>> I don't believe there is need for the loop either.
>> Any other updater will also be including our allocated pages
>> (and at this point in the code eliminated from roll back)
>> so if they beat us to it, then no problem, their max is better than ours.
>
> Let's assume we don't have the loop.
>
>
> CPU A CPU B
>
> alloced_pages = 2001
> old_max = cur_max = 2000
> alloced_pages = 2005
> old_max = cur_max = 2000
>
> cmpxchg(2000, 2000, 2001) -> OK
>
> cmpxchg(2001, 2000, 2005) -> FAIL
>
> So, we lose 2005 which is bigger vaule.
>

Yes - you are absolutely correct - I missed that scenario.

but there isn't the need to redo zs_get_total_size.

we only need to loop while our value is still the max.

So the two parts are not closely coupled and the inline code for the
exceeded check is simple enough.
And the loop to apply max would be best in helper function.

>>
>>
>>
>> > static int zram_bvec_write(struct zram *zram, struct bio_vec *bvec, u32 index,
>> > int offset)
>> > {
>> > @@ -541,8 +595,7 @@ static int zram_bvec_write(struct zram *zram, struct bio_vec *bvec, u32 index,
>> > goto out;
>> > }
>> >
>> > - if (zram->limit_pages &&
>> > - zs_get_total_size(meta->mem_pool) > zram->limit_pages) {
>> > + if (!check_limit(zram)) {
>> > zs_free(meta->mem_pool, handle);
>> > ret = -ENOMEM;
>> > goto out;
>> > @@ -897,6 +950,8 @@ static DEVICE_ATTR(orig_data_size, S_IRUGO, orig_data_size_show, NULL);
>> > static DEVICE_ATTR(mem_used_total, S_IRUGO, mem_used_total_show, NULL);
>> > static DEVICE_ATTR(mem_limit, S_IRUGO | S_IWUSR, mem_limit_show,
>> > mem_limit_store);
>> > +static DEVICE_ATTR(mem_used_max, S_IRUGO | S_IWUSR, mem_used_max_show,
>> > + mem_used_max_store);
>> > static DEVICE_ATTR(max_comp_streams, S_IRUGO | S_IWUSR,
>> > max_comp_streams_show, max_comp_streams_store);
>> > static DEVICE_ATTR(comp_algorithm, S_IRUGO | S_IWUSR,
>> > @@ -926,6 +981,7 @@ static struct attribute *zram_disk_attrs[] = {
>> > &dev_attr_compr_data_size.attr,
>> > &dev_attr_mem_used_total.attr,
>> > &dev_attr_mem_limit.attr,
>> > + &dev_attr_mem_used_max.attr,
>> > &dev_attr_max_comp_streams.attr,
>> > &dev_attr_comp_algorithm.attr,
>> > NULL,
>> > diff --git a/drivers/block/zram/zram_drv.h b/drivers/block/zram/zram_drv.h
>> > index b7aa9c21553f..29383312d543 100644
>> > --- a/drivers/block/zram/zram_drv.h
>> > +++ b/drivers/block/zram/zram_drv.h
>> > @@ -90,6 +90,7 @@ struct zram_stats {
>> > atomic64_t notify_free; /* no. of swap slot free notifications */
>> > atomic64_t zero_pages; /* no. of zero filled pages */
>> > atomic64_t pages_stored; /* no. of pages currently stored */
>> > + atomic64_t max_used_pages; /* no. of maximum pages stored */
>> > };
>> >
>> > struct zram_meta {
>> > --
>> > 2.0.0
>> >
>>
>> --
>> To unsubscribe, send a message with 'unsubscribe linux-mm' in
>> the body to majordomo@xxxxxxxxxx For more info on Linux MM,
>> see: http://www.linux-mm.org/ .
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>
> --
> Kind regards,
> Minchan Kim
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Next message: Thierry Reding: "Re: [PATCH 2/3] ARM: at91/tclib: move initialization from alloc to probe"
Previous message: Jani Nikula: "Re: WARNING in 3.17-rc1 for i915"
In reply to: Minchan Kim: "Re: [PATCH v2 4/4] zram: report maximum used memory"
Next in thread: Minchan Kim: "Re: [PATCH v2 4/4] zram: report maximum used memory"
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