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From:   Thomas Gleixner <tglx@linutronix.de>
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To:     Peter Zijlstra <peterz@infradead.org>,
        Peter Oskolkov <posk@posk.io>
Cc:     Ingo Molnar <mingo@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Andy Lutomirski <luto@kernel.org>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, linux-api@vger.kernel.org,
        Paul Turner <pjt@google.com>, Ben Segall <bsegall@google.com>,
        Peter Oskolkov <posk@google.com>,
        Andrei Vagin <avagin@google.com>, Jann Horn <jannh@google.com>,
        Thierry Delisle <tdelisle@uwaterloo.ca>
Subject: Re: [PATCH v0.9.1 3/6] sched/umcg: implement UMCG syscalls
In-Reply-To: <20211124211927.GG721624@worktop.programming.kicks-ass.net>
References: <20211122211327.5931-1-posk@google.com>
 <20211122211327.5931-4-posk@google.com>
 <20211124211927.GG721624@worktop.programming.kicks-ass.net>
Date:   Fri, 26 Nov 2021 22:11:17 +0100
Message-ID: <877dcuhbbe.ffs@tglx>
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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