Date: Thu, 2 Apr 2015 17:34:20 +0300 From: Gleb Smirnoff <glebius@FreeBSD.org> To: Mateusz Guzik <mjguzik@gmail.com> Cc: svn-src-head@freebsd.org, svn-src-all@freebsd.org, src-committers@freebsd.org, Ian Lepore <ian@freebsd.org> Subject: Re: svn commit: r280971 - in head: contrib/ipfilter/tools share/man/man4 sys/contrib/ipfilter/netinet sys/netinet sys/netipsec sys/netpfil/pf Message-ID: <20150402143420.GI64665@FreeBSD.org> In-Reply-To: <20150402142318.GC549@dft-labs.eu> References: <201504012226.t31MQedN044443@svn.freebsd.org> <1427929676.82583.103.camel@freebsd.org> <20150402123522.GC64665@FreeBSD.org> <20150402133751.GA549@dft-labs.eu> <20150402134217.GG64665@FreeBSD.org> <20150402135157.GB549@dft-labs.eu> <1427983109.82583.115.camel@freebsd.org> <20150402142318.GC549@dft-labs.eu>
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On Thu, Apr 02, 2015 at 04:23:18PM +0200, Mateusz Guzik wrote: M> Well, a thread migrating to another cpu is the standard thing everyone M> sees. How often do you see it right in a particular window of 2 or 3 instructions? If you carefully read the thread I referred to, you would notice that on many arches, save amd64 and i386, all systems stats are prone to mangling the stats due to migration within PCPU_INC. Look here: grep '^#define PCPU_ADD' sys/*/include/pcpu.h Do we have reports on not precise enough statistics, yet? M> I don't feel that strongly about changing the code. If it stays as it M> is, it definitely needs the comment I mentioned explaining why migration M> is fine. I just added the comment. M> If the code was to be changed a machdep counter_u64_fetchadd seems like M> the only course of action. If we have more places in kernel, where such API is required, I would do it. Alas, seems like not possible to make it without critical section, even on amd64. Note that counter_u64_add() on amd64 is critical-less. -- Totus tuus, Glebius.
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