linux-kernel - Re: [PATCH v0.9.1 3/6] sched/umcg: implement UMCG syscalls

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Date:   Fri, 26 Nov 2021 22:11:17 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Peter Zijlstra <peterz@...radead.org>,
        Peter Oskolkov <posk@...k.io>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, linux-api@...r.kernel.org,
        Paul Turner <pjt@...gle.com>, Ben Segall <bsegall@...gle.com>,
        Peter Oskolkov <posk@...gle.com>,
        Andrei Vagin <avagin@...gle.com>, Jann Horn <jannh@...gle.com>,
        Thierry Delisle <tdelisle@...terloo.ca>
Subject: Re: [PATCH v0.9.1 3/6] sched/umcg: implement UMCG syscalls

On Wed, Nov 24 2021 at 22:19, Peter Zijlstra wrote:
> On Mon, Nov 22, 2021 at 01:13:24PM -0800, Peter Oskolkov wrote:
>
>> +	 * Timestamp: a 46-bit CLOCK_MONOTONIC timestamp, at 16ns resolution.
>
>> +static int umcg_update_state(u64 __user *state_ts, u64 *expected, u64 desired,
>> +				bool may_fault)
>> +{
>> +	u64 curr_ts = (*expected) >> (64 - UMCG_STATE_TIMESTAMP_BITS);
>> +	u64 next_ts = ktime_get_ns() >> UMCG_STATE_TIMESTAMP_GRANULARITY;
>
> I'm still very hesitant to use ktime (fear the HPET); but I suppose it
> makes sense to use a time base that's accessible to userspace. Was
> MONOTONIC_RAW considered?

MONOTONIC_RAW is not really useful as you can't sleep on it and it won't
solve the HPET crap either.

Thanks,

        tglx