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Date:   Tue, 27 Aug 2019 23:50:35 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Matthew Garrett <mjg59@srcf.ucam.org>
Cc:     Vineeth Remanan Pillai <vpillai@digitalocean.com>,
        Nishanth Aravamudan <naravamudan@digitalocean.com>,
        Julien Desfossez <jdesfossez@digitalocean.com>,
        Tim Chen <tim.c.chen@linux.intel.com>, mingo@kernel.org,
        tglx@linutronix.de, pjt@google.com, torvalds@linux-foundation.org,
        linux-kernel@vger.kernel.org, subhra.mazumdar@oracle.com,
        fweisbec@gmail.com, keescook@chromium.org, kerrnel@google.com,
        Phil Auld <pauld@redhat.com>, Aaron Lu <aaron.lwe@gmail.com>,
        Aubrey Li <aubrey.intel@gmail.com>,
        Valentin Schneider <valentin.schneider@arm.com>,
        Mel Gorman <mgorman@techsingularity.net>,
        Pawan Gupta <pawan.kumar.gupta@linux.intel.com>,
        Paolo Bonzini <pbonzini@redhat.com>
Subject: Re: [RFC PATCH v3 00/16] Core scheduling v3
Message-ID: <20190827215035.GH2332@hirez.programming.kicks-ass.net>
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 <20190827211417.snpwgnhsu5t6u52y@srcf.ucam.org>
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On Tue, Aug 27, 2019 at 10:14:17PM +0100, Matthew Garrett wrote:
> Apple have provided a sysctl that allows applications to indicate that 
> specific threads should make use of core isolation while allowing 
> the rest of the system to make use of SMT, and browsers (Safari, Firefox 
> and Chrome, at least) are now making use of this. Trying to do something 
> similar using cgroups seems a bit awkward. Would something like this be 
> reasonable? 

Sure; like I wrote earlier; I only did the cgroup thing because I was
lazy and it was the easiest interface to hack on in a hurry.

The rest of the ABI nonsense can 'trivially' be done later; if when we
decide to actually do this.

And given MDS, I'm still not entirely convinced it all makes sense. If
it were just L1TF, then yes, but now...

> Having spoken to the Chrome team, I believe that the 
> semantics we want are:
> 
> 1) A thread to be able to indicate that it should not run on the same 
> core as anything not in posession of the same cookie
> 2) Descendents of that thread to (by default) have the same cookie
> 3) No other thread be able to obtain the same cookie
> 4) Threads not be able to rejoin the global group (ie, threads can 
> segregate themselves from their parent and peers, but can never rejoin 
> that group once segregated)
> 
> but don't know if that's what everyone else would want.
> 
> diff --git a/include/uapi/linux/prctl.h b/include/uapi/linux/prctl.h
> index 094bb03b9cc2..5d411246d4d5 100644
> --- a/include/uapi/linux/prctl.h
> +++ b/include/uapi/linux/prctl.h
> @@ -229,4 +229,5 @@ struct prctl_mm_map {
>  # define PR_PAC_APDBKEY			(1UL << 3)
>  # define PR_PAC_APGAKEY			(1UL << 4)
>  
> +#define PR_CORE_ISOLATE			55
>  #endif /* _LINUX_PRCTL_H */
> diff --git a/kernel/sys.c b/kernel/sys.c
> index 12df0e5434b8..a054cfcca511 100644
> --- a/kernel/sys.c
> +++ b/kernel/sys.c
> @@ -2486,6 +2486,13 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
>  			return -EINVAL;
>  		error = PAC_RESET_KEYS(me, arg2);
>  		break;
> +	case PR_CORE_ISOLATE:
> +#ifdef CONFIG_SCHED_CORE
> +		current->core_cookie = (unsigned long)current;

This needs to then also force a reschedule of current. And there's the
little issue of what happens if 'current' dies while its children live
on, and current gets re-used for a new process and does this again.

> +#else
> +		result = -EINVAL;
> +#endif
> +		break;
>  	default:
>  		error = -EINVAL;
>  		break;
> 
> 
> -- 
> Matthew Garrett | mjg59@srcf.ucam.org