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Re: rhashtable: Use __vmalloc with GFP_ATOMIC for table allocation

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On Fri, Dec 04, 2015 at 09:45:20AM -0800, Eric Dumazet wrote:
> On Fri, 2015-12-04 at 18:01 +0100, Phil Sutter wrote:
> > On Fri, Dec 04, 2015 at 10:39:56PM +0800, Herbert Xu wrote:
> > > On Thu, Dec 03, 2015 at 08:08:39AM -0800, Eric Dumazet wrote:
> > > >
> > > > Anyway, __vmalloc() can be used with GFP_ATOMIC, have you tried this ?
> > > 
> > > OK I've tried it and I no longer get any ENOMEM errors!
> > 
> > I can't confirm this, sadly. Using 50 threads, results seem to be stable
> > and good. But increasing the number of threads I can provoke ENOMEM
> > condition again. See attached log which shows a failing test run with
> > 100 threads.
> > 
> > I tried to extract logs of a test run with as few as possible failing
> > threads, but wasn't successful. It seems like the error amplifies
> > itself: While having stable success with less than 70 threads, going
> > beyond a margin I could not identify exactly, much more threads failed
> > than expected. For instance, the attached log shows 70 out of 100
> > threads failing, while for me every single test with 50 threads was
> > successful.
> 
> But this patch is about GFP_ATOMIC allocations, I doubt your test is
> using GFP_ATOMIC.
> 
> Threads (process context) should use GFP_KERNEL allocations.

Well, I assumed Herbert did his tests using test_rhashtable, and
therefore fixed whatever code-path that triggers. Maybe I'm wrong,
though.

Looking at the vmalloc allocation failure trace, it seems like it's
trying to indeed use GFP_ATOMIC from inside those threads: If I don't
miss anything, bucket_table_alloc is called from
rhashtable_insert_rehash, which passes GFP_ATOMIC unconditionally. But
then again bucket_table_alloc should use kzalloc if 'gfp != GFP_KERNEL',
so I'm probably just cross-eyed right now.

> BTW, if 100 threads are simultaneously trying to vmalloc(32 MB), this
> might not be very wise :(
> 
> Only one should really do this, while others are waiting.

Sure, that was my previous understanding of how this thing works.

> If we really want parallelism (multiple cpus coordinating their effort),
> it should be done very differently.

Maybe my approach of stress-testing rhashtable was too naive in the
first place.

Thanks, Phil
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rhashtable: ENOMEM errors when hit with a flood of insertions Herbert Xu <herbert@gondor.apana.org.au> - 2015-12-03 14:00 +0100
  RE: rhashtable: ENOMEM errors when hit with a flood of insertions David Laight <David.Laight@ACULAB.COM> - 2015-12-03 16:20 +0100
  Re: rhashtable: ENOMEM errors when hit with a flood of insertions Eric Dumazet <eric.dumazet@gmail.com> - 2015-12-03 17:10 +0100
    Re: rhashtable: ENOMEM errors when hit with a flood of insertions Herbert Xu <herbert@gondor.apana.org.au> - 2015-12-04 01:10 +0100
    rhashtable: Use __vmalloc with GFP_ATOMIC for table allocation Herbert Xu <herbert@gondor.apana.org.au> - 2015-12-04 15:50 +0100
      Re: rhashtable: Use __vmalloc with GFP_ATOMIC for table allocation Eric Dumazet <eric.dumazet@gmail.com> - 2015-12-04 18:50 +0100
        Re: rhashtable: Use __vmalloc with GFP_ATOMIC for table allocation Phil Sutter <phil@nwl.cc> - 2015-12-04 19:20 +0100
          Re: rhashtable: Use __vmalloc with GFP_ATOMIC for table allocation Herbert Xu <herbert@gondor.apana.org.au> - 2015-12-05 08:10 +0100
            Re: rhashtable: Use __vmalloc with GFP_ATOMIC for table allocation Thomas Graf <tgraf@suug.ch> - 2015-12-07 16:40 +0100
              Re: rhashtable: Use __vmalloc with GFP_ATOMIC for table allocation David Miller <davem@davemloft.net> - 2015-12-07 20:40 +0100
                Re: rhashtable: Use __vmalloc with GFP_ATOMIC for table allocation Thomas Graf <tgraf@suug.ch> - 2015-12-09 03:20 +0100
                Re: rhashtable: Use __vmalloc with GFP_ATOMIC for table allocation Herbert Xu <herbert@gondor.apana.org.au> - 2015-12-09 03:30 +0100
                Re: rhashtable: Use __vmalloc with GFP_ATOMIC for table allocation Herbert Xu <herbert@gondor.apana.org.au> - 2015-12-09 03:40 +0100
                Re: rhashtable: Use __vmalloc with GFP_ATOMIC for table allocation Thomas Graf <tgraf@suug.ch> - 2015-12-09 03:50 +0100
                Re: rhashtable: Use __vmalloc with GFP_ATOMIC for table allocation Thomas Graf <tgraf@suug.ch> - 2015-12-09 03:40 +0100
      Re: rhashtable: Use __vmalloc with GFP_ATOMIC for table allocation David Miller <davem@davemloft.net> - 2015-12-04 23:00 +0100
        Re: rhashtable: Use __vmalloc with GFP_ATOMIC for table allocation Herbert Xu <herbert@gondor.apana.org.au> - 2015-12-05 08:10 +0100
          Re: rhashtable: Use __vmalloc with GFP_ATOMIC for table allocation David Miller <davem@davemloft.net> - 2015-12-06 04:50 +0100

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