linux-kernel - Re: Re: [PATCH v3 2/2] zram: try vmalloc() after kmalloc()

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Date:	Tue, 1 Dec 2015 11:04:09 +0900
From:	"kyeongdon.kim" <kyeongdon.kim@....com>
To:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:	Minchan Kim <minchan@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org,
	Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Subject: Re: Re: [PATCH v3 2/2] zram: try vmalloc() after kmalloc()

On 2015-11-30 오후 8:14, Sergey Senozhatsky wrote:
> On (11/30/15 19:42), kyeongdon.kim wrote:
> [..]
>> Sorry to have kept you waiting,
>> Obviously, I couldn't see allocation fail message with this patch.
>> But, there is something to make some delay(not sure yet this is normal).
> 
> what delay? how significant it is? do you see it in practice or it's just
> a guess?
> 
Now, I just checked with printk() log in zcomp_lz4_create()/lzo_create()
to see address 'ret'. and these 'null' values are called several times
from kzalloc. also __vmalloc. - these are making the delay.
So, not significant status I guess. but if this allocation try is many.
I doubt is is fine.

>> static struct zcomp_strm *zcomp_strm_alloc(struct zcomp *comp)
>> {
>> <snip>
>>
>> zstrm->private = comp->backend->create();
>> ^ // sometimes, return 'null' continually(2-5times)
>>
>> As you know, if there is 'null' return, this function is called again to
>> get a memory in while() loop. I just checked this one with printk().
> 
> well, not always.
> 
> a) current wait_event() for available stream to become idle.
> b) once current awaken it attempts to get an idle stream
> c) if zstrm then return
> d) if there is no idle stream then goto a)
> e) else try to allocate stream again, if !zstrm goto a), else return
> 
> while (1) {
> spin_lock(&zs->strm_lock);
> if (!list_empty(&zs->idle_strm)) {
> zstrm = list_entry(zs->idle_strm.next,
> struct zcomp_strm, list);
> list_del(&zstrm->list);
> spin_unlock(&zs->strm_lock);
> return zstrm;
> }
> /* zstrm streams limit reached, wait for idle stream */
> if (zs->avail_strm >= zs->max_strm) {
> spin_unlock(&zs->strm_lock);
> wait_event(zs->strm_wait, !list_empty(&zs->idle_strm));
> continue;
> }
> /* allocate new zstrm stream */
> zs->avail_strm++;
> spin_unlock(&zs->strm_lock);
> 
> zstrm = zcomp_strm_alloc(comp);
> if (!zstrm) {
> spin_lock(&zs->strm_lock);
> zs->avail_strm--;
> spin_unlock(&zs->strm_lock);
> wait_event(zs->strm_wait, !list_empty(&zs->idle_strm));
> continue;
> }
> break;
> }
> 
> so it's possible for current to zcomp_strm_alloc() several times...
> 
> do you see the same process doing N zcomp_strm_alloc() calls, or it's N
> processes
> doing one zcomp_strm_alloc()? I think the latter one is more likely;
> once we failed
> to zcomp_strm_alloc() quite possible that N concurrent or succeeding IOs
> will do
> the same. That's why I proposed to decrease ->max_strm; but we basically
> don't know
> when we shall rollback it to the original value; I'm not sure I want to
> do something
> like: every 42nd IO try to increment ->max_strm by one, until it's less
> than the
> original value.
> 
> so I'd probably prefer to keep it the way it is; but let's see the
> numbers from
> you first.
> 
> -ss

I didn't check detailed yet.I'll explain after checking this.

Thanks,
Kyeongdon Kim

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