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Date: Mon, 6 Oct 2014 14:28:26 +0200 (CEST) From: Thomas Gleixner <tglx@...utronix.de> To: David Lang <david@...g.hm> cc: Christoph Lameter <cl@...ux.com>, Richard Cochran <richardcochran@...il.com>, linux-kernel@...r.kernel.org Subject: Re: Why do we still have 32 bit counters? Interrupt counters overflow within 50 days On Mon, 6 Oct 2014, David Lang wrote: > On Sun, 5 Oct 2014, Thomas Gleixner wrote: > > > > > > > 64 bit counters on 32 bit machines are not an easy thing and could be > > > > Whats so hard about 64bit counters on 32bit machines? > > > > > expensive to handle in particular because these counters are used in > > > performance critical hotpaths. > > > > The expensive overhead is a single "adcl" instruction. > > As I understand it, since the 64 bit math cannot be made atomic, it requires > protecting the counter with a lock so that it can't be read while half > updated. Aquiring a lock on every update is an expensive thing to do. It's not > something people like to see in a fast path, especially for something of as > low an importance as the counters. As I said before. Both reader and writer side are already protected by the irq descriptor lock. We take that lock anyway. Thanks, tglx -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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