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Date: Sat, 7 Sep 2019 20:00:34 +0900 From: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp> To: Michal Hocko <mhocko@...nel.org>, Sergey Senozhatsky <sergey.senozhatsky.work@...il.com> 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 2019/09/04 17:25, Michal Hocko wrote: > On Wed 04-09-19 16:00:42, Sergey Senozhatsky wrote: >> On (09/04/19 15:41), Sergey Senozhatsky wrote: >>> 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). >> >> And on devices with slow serial consoles this can be somewhat close to >> "no ratelimit". *Suppose* that warn_alloc() adds 700 lines each time. >> Within 5 seconds we can call warn_alloc() 10 times, which will add 7000 >> lines to the logbuf. If printk() can evict only 6000 lines in 5 seconds >> then we have a growing number of pending logbuf messages. > > Yes, ratelimit is problematic when the ratelimited operation is slow. I > guess that is a well known problem and we would need to rework both the > api and the implementation to make it work in those cases as well. > Essentially we need to make the ratelimit act as a gatekeeper to an > operation section - something like a critical section except you can > tolerate more code executions but not too many. So effectively > > start_throttle(rate, number); > /* here goes your operation */ > end_throttle(); > > one operation is not considered done until the whole section ends. > Or something along those lines. Regarding OOM killer which is serialized by oom_lock mutex, I proposed "mm, oom: More aggressively ratelimit dump_header()." at https://lkml.kernel.org/r/1550325895-9291-2-git-send-email-penguin-kernel@I-love.SAKURA.ne.jp but it was ignored. > > In this particular case we can increase the rate limit parameters of > course but I think that longterm we need a better api. > I proposed "printk: Introduce "store now but print later" prefix." at https://lkml.kernel.org/r/1550896930-12324-1-git-send-email-penguin-kernel@I-love.SAKURA.ne.jp but it was not accepted. But I think that more better solution for warn_alloc() is to defer printing from allocating context (which may not be allowed to sleep) to WQ context. I proposed "mm,oom: Defer dump_tasks() output." at https://lkml.kernel.org/r/7de2310d-afbd-e616-e83a-d75103b986c6@i-love.sakura.ne.jp and answered to Michal's concerns. I hope we can apply the same thing for warn_alloc() and show_mem(). Then, we can minimize latency for both "memory allocation failures" and "OOM killer invocations".
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