linux-kernel - Re: 2048 CPUs [was: Re: New filesystem for Linux]

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]

Date:	Tue, 7 Nov 2006 23:36:19 +0100 (CET)
From:	Mikulas Patocka <mikulas@...ax.karlin.mff.cuni.cz>
To:	Pavel Machek <pavel@....cz>
Cc:	Albert Cahalan <acahalan@...il.com>, linux-kernel@...r.kernel.org
Subject: Re: 2048 CPUs [was: Re: New filesystem for Linux]

Hi!

> Hi!
>
>>> The SGI Altix can have 2048 CPUs.
>>
>> And does it run one image of Linux? Or more images each
>> on few cpus?
>>
>> How do they solve problem with spinlock livelocks?
>>
>> If time-spent-outside-spinlock/time-spent-in-spinlock <
>> number-of-cpus, the spinlock livelock may happen ---
>> this condition is not true normally with 2 or 4 cpus,
>> but for that high amount of cpus, there is a danger.
>
> Lets say time-spent-outside-spinlock == time-spent-in-spinlock and
> number-of-cpus == 2.
>
> 1 < 2 , so it should livelock according to you...

There is off-by-one bug in the condition. It should be:
(time_spent_in_spinlock + time_spent_outside_spinlock) / 
time_spent_in_spinlock < number_of_cpus

... or if you divide it by time_spent_in_spinlock:
time_spent_outside_spinlock / time_spent_in_spinlock + 1 < number_of_cpus

> ...but afaict this should work okay. Even if spinlocks are very
> unfair, as long as time-outside and time-inside comes in big chunks,
> it should work.
>
> If you are unlucky, one cpu may stall for a while, but... I see no
> livelock.

If some rogue threads (and it may not even be intetional) call the same 
syscall stressing the one spinlock all the time, other syscalls needing 
the same spinlock may stall.

Maybe there are so few Altices in the world that no one has yet observed 
it...

Mikulas
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/