linux-kernel - Re: [PATCH 1/2] perf/x86/intel: enable CPU ref

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite for Android: free password hash cracker in your pocket

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20170530174014.zjauj22hx7avxqgf@hirez.programming.kicks-ass.net>
Date:   Tue, 30 May 2017 19:40:14 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Andi Kleen <ak@...ux.intel.com>
Cc:     Stephane Eranian <eranian@...gle.com>,
        Vince Weaver <vincent.weaver@...ne.edu>,
        "Liang, Kan" <kan.liang@...el.com>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "alexander.shishkin@...ux.intel.com" 
        <alexander.shishkin@...ux.intel.com>,
        "acme@...hat.com" <acme@...hat.com>,
        "jolsa@...hat.com" <jolsa@...hat.com>,
        "torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>
Subject: Re: [PATCH 1/2] perf/x86/intel: enable CPU ref_cycles for GP counter

On Tue, May 30, 2017 at 10:22:08AM -0700, Andi Kleen wrote:
> > > You would only need a single one per system however, not one per CPU.
> > > RCU already tracks all the CPUs, all we need is a single NMI watchdog
> > > that makes sure RCU itself does not get stuck.
> > > 
> > > So we just have to find a single watchdog somewhere that can trigger
> > > NMI.
> > 
> > But then you have to IPI broadcast the NMI, which is less than ideal.
> 
> Only when the watchdog times out to print the backtraces.

The current NMI watchdog has a per-cpu state. So that means either doing
for_all_cpu() loops or IPI broadcasts from the NMI tickle. Neither is
something you really want.

> > RCU doesn't have that problem because the quiescent state is a global
> > thing. CPU progress, which is what the NMI watchdog tests, is very much
> > per logical CPU though.
> 
> RCU already has a CPU stall detector. It should work (and usually
> triggers before the NMI watchdog in my experience unless the
> whole system is dead)

It only goes look at CPU state once it detects the global QS is stalled
I think. But I've not had much luck with the RCU one -- although I think
its been improved since I last had a hard problem.