netdev - Re: [PATCH] net/skbuff: silence warnings under memory pressure

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Date:   Wed, 4 Sep 2019 15:41:44 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Qian Cai <cai@....pw>, Eric Dumazet <eric.dumazet@...il.com>,
        davem@...emloft.net, netdev@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH] net/skbuff: silence warnings under memory pressure

On (09/04/19 08:15), Michal Hocko wrote:
> > If you look at the original report, the failed allocation dump_stack() is,
> > 
> >  <IRQ>
> >  warn_alloc.cold.43+0x8a/0x148
> >  __alloc_pages_nodemask+0x1a5c/0x1bb0
> >  alloc_pages_current+0x9c/0x110
> >  allocate_slab+0x34a/0x11f0
> >  new_slab+0x46/0x70
> >  ___slab_alloc+0x604/0x950
> >  __slab_alloc+0x12/0x20
> >  kmem_cache_alloc+0x32a/0x400
> >  __build_skb+0x23/0x60
> >  build_skb+0x1a/0xb0
> >  igb_clean_rx_irq+0xafc/0x1010 [igb]
> >  igb_poll+0x4bb/0xe30 [igb]
> >  net_rx_action+0x244/0x7a0
> >  __do_softirq+0x1a0/0x60a
> >  irq_exit+0xb5/0xd0
> >  do_IRQ+0x81/0x170
> >  common_interrupt+0xf/0xf
> >  </IRQ>
> > 
> > Since it has no __GFP_NOWARN to begin with, it will call,

I think that DEFAULT_RATELIMIT_INTERVAL and DEFAULT_RATELIMIT_BURST
are good when we ratelimit just a single printk() call, so the ratelimit
is "max 10 kernel log lines in 5 seconds".

But the thing is different in case of dump_stack() + show_mem() +
some other output. Because now we ratelimit not a single printk() line,
but hundreds of them. The ratelimit becomes - 10 * $$$ lines in 5 seconds
(IOW, now we talk about thousands of lines). Significantly more permissive
ratelimiting.

	-ss