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Date:   Wed, 24 Nov 2021 22:19:27 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Peter Oskolkov <posk@posk.io>
Cc:     Ingo Molnar <mingo@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Andrew Morton <akpm@linux-foundation.org>,
        Dave Hansen <dave.hansen@linux.intel.com>,
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        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
Message-ID: <20211124211927.GG721624@worktop.programming.kicks-ass.net>
References: <20211122211327.5931-1-posk@google.com>
 <20211122211327.5931-4-posk@google.com>
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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?