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From: Josh Poimboeuf <jpoimboe@redhat.com>
To: speck@linutronix.de
Subject: [MODERATED] Re: [PATCH 0/2] more sampling fun 0
Date: Thu, 20 Feb 2020 15:51:32 -0600	[thread overview]
Message-ID: <20200220215132.vl6mrauahldbzkfm@treble> (raw)
In-Reply-To: <20200220145510.GE160988@tassilo.jf.intel.com>

On Thu, Feb 20, 2020 at 06:55:10AM -0800, speck for Andi Kleen wrote:
> > Then we need to stop using RDRAND internally for our "give me a random
> > number api" which has spread to more and more parts of the kernel.
> 
> Only if that API is called frequently enough. AFAIK it is not. 
> 
> Normally it's used for rare rekeying of hash tables etc., which
> doesn't happen very often.
> 
> > Here's a patch that does so:
> > 	https://lore.kernel.org/lkml/20200216161836.1976-1-Jason@zx2c4.com/
> > which I'm going to advise get merged now and backported to the stable
> > branches.
> 
> Don't see any reason at this point. Only do it if there's an actual
> indication of a problem.

Internal testing of the SRBDS beta microcode on Kaby Lake is showing
significant slowdowns in several syscall microbenchmarks.

One pthread_create() microbenchmark had a ~48% slowdown.  We confirmed
it was due to RDRAND in get_random_u64().

In this case I think the path was:

 clone()
   _do_fork()
     copy_process()
       dup_task_struct()
         get_random_canary() (due to CONFIG_STACKPROTECTOR)
	   get_random_long()
	     get_random_u64()
	       arch_get_random_long()
	         RDRAND

-- 
Josh

  parent reply	other threads:[~2020-02-20 21:51 UTC|newest]

Thread overview: 21+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-02-19 22:45 [MODERATED] " mark gross
2020-02-20  1:53 ` [MODERATED] " mark gross
2020-02-20  8:14 ` Greg KH
2020-02-20 14:27   ` Greg KH
2020-02-20 15:40     ` mark gross
2020-02-20 16:18       ` Greg KH
2020-02-20 14:55   ` Andi Kleen
2020-02-20 15:05     ` Greg KH
2020-02-20 16:55       ` Andi Kleen
2020-02-20 21:51     ` Josh Poimboeuf [this message]
2020-02-20 22:15       ` Andi Kleen
2020-02-20 22:59         ` Kees Cook
2020-02-20 15:09   ` mark gross
2020-02-28 16:21   ` [MODERATED] Additional sampling fun Borislav Petkov
2020-02-28 16:34     ` [MODERATED] " Greg KH
2020-02-28 17:38       ` mark gross
2020-02-28 17:44         ` Thomas Gleixner
2020-02-28 18:09           ` [MODERATED] " Luck, Tony
2020-02-28 18:40             ` Borislav Petkov
2020-02-28 21:53             ` Thomas Gleixner
2020-03-03  1:03               ` [MODERATED] " Luck, Tony

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