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

[Minchan Kim]

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.

> 
> 
> 
> >  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
> >
> 
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-- 
Kind regards,
Minchan Kim
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