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Message-ID: <20140820075326.GC17372@bbox>
Date: Wed, 20 Aug 2014 16:53:26 +0900
From: Minchan Kim <minchan@...nel.org>
To: David Horner <ds2horner@...il.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
Linux-MM <linux-mm@...ck.org>,
linux-kernel <linux-kernel@...r.kernel.org>,
Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
Jerome Marchand <jmarchan@...hat.com>, juno.choi@....com,
seungho1.park@....com, Luigi Semenzato <semenzato@...gle.com>,
Nitin Gupta <ngupta@...are.org>,
Seth Jennings <sjennings@...iantweb.net>,
Dan Streetman <ddstreet@...e.org>
Subject: Re: [PATCH v2 4/4] zram: report maximum used memory
On Wed, Aug 20, 2014 at 03:38:27AM -0400, David Horner wrote:
> On Wed, Aug 20, 2014 at 2:53 AM, Minchan Kim <minchan@...nel.org> 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@...nel.org> 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@...nel.org>
> >> > ---
> >> > 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@...nel.org>
> >> > +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@...nel.org>
> >> > 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.
Yes - you are absolutely right. :)
>
> 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.
Okay, you proved helper function would be better for readabilty
to indicate limit check and max_used_check is not coupled.
Thanks for the review, David!
>
> >>
> >>
> >>
> >> > 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@...ck.org. For more info on Linux MM,
> >> see: http://www.linux-mm.org/ .
> >> Don't email: <a href=mailto:"dont@...ck.org"> email@...ck.org </a>
> >
> > --
> > Kind regards,
> > Minchan Kim
>
> --
> To unsubscribe, send a message with 'unsubscribe linux-mm' in
> the body to majordomo@...ck.org. For more info on Linux MM,
> see: http://www.linux-mm.org/ .
> Don't email: <a href=mailto:"dont@...ck.org"> email@...ck.org </a>
--
Kind regards,
Minchan Kim
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