Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Thu, 18 Jul 2019 15:21:08 +0200
From:   Oleg Nesterov <oleg@redhat.com>
To:     Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>
Cc:     Andrew Fox <afox@redhat.com>,
        Stephen Johnston <sjohnsto@redhat.com>,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] sched/cputime: make scale_stime() more precise
Message-ID: <20190718132108.GA22220@redhat.com>
References: <20190718131834.GA22211@redhat.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20190718131834.GA22211@redhat.com>
User-Agent: Mutt/1.5.24 (2015-08-30)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

To simplify the review, see the code with this patch applied:

/*
 * Perform (stime * rtime) / total, but avoid multiplication overflow
 * by losing precision when the numbers are big.
 *
 * NOTE! currently the only user is cputime_adjust() and thus
 *
 *	stime < total && rtime > total
 *
 * this means that the end result is always precise and the additional
 * div64_u64_rem() inside the main loop is called at most once.
 */
static u64 scale_stime(u64 stime, u64 rtime, u64 total)
{
	u64 res = 0, div, rem;

	/* can stime * rtime overflow ? */
	while (ilog2(stime) + ilog2(rtime) > 62) {
		if (stime > rtime)
			swap(rtime, stime);

		if (rtime >= total) {
			/*
			 * (rtime * stime) / total is equal to
			 *
			 *	(rtime / total) * stime +
			 *	(rtime % total) * stime / total
			 *
			 * if nothing overflows. Can the 1st multiplication
			 * overflow? Yes, but we do not care: this can only
			 * happen if the end result can't fit in u64 anyway.
			 *
			 * So the code below does
			 *
			 *	res += (rtime / total) * stime;
			 *	rtime = rtime % total;
			 */
			div = div64_u64_rem(rtime, total, &rem);
			res += div * stime;
			rtime = rem;
			continue;
		}

		/* drop precision */
		rtime >>= 1;
		total >>= 1;
		if (!total)
			return res;
	}

	return res + div64_u64(stime * rtime, total);
}