Date: Fri, 27 Feb 2015 07:50:55 -0800 From: Harrison Grundy <harrison.grundy@astrodoggroup.com> To: freebsd-arch@freebsd.org Subject: Re: Minor ULE changes and optimizations Message-ID: <54F0925F.30002@astrodoggroup.com> In-Reply-To: <2311645.BNIPBaFv2E@ralph.baldwin.cx> References: <54EF2C54.7030207@astrodoggroup.com> <2311645.BNIPBaFv2E@ralph.baldwin.cx>
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On 02/27/15 06:14, John Baldwin wrote: > On Thursday, February 26, 2015 06:23:16 AM Harrison Grundy wrote: >> https://reviews.freebsd.org/D1969 This allows a non-migratable >> thread to pin itself to a CPU if it is already running on that >> CPU. >> >> I've been running these patches for the past week or so without >> issue. Any additional testing or comments would be greatly >> appreciated. > > Can you explain the reason / use case for this? This seems to be > allowing an API violation. sched_pin() was designed to be a > lower-level API than sched_bind(), so you wouldn't call > sched_bind() if you were already pinned. In addition, sched_pin() > is sometimes used by code that assumes it won't migrate until > sched_unpin() (e.g. temporary mappings inside an sfbuf). If you > allow sched_bind() to move a thread that is pinned you will allow > someone to unintentionally break those sort of things instead of > getting an assertion failure panic. > For a pinned thread, the underlying idea is that if you're already on the CPU you pinned to, calling sched_bind with that CPU specified allows you to set TSF_BOUND without calling sched_unpin first. If a pinned thread were to call sched_bind for a CPU it isn't pinned to, it would still hit the assert and fail. For any unpinned thread, if you're already running on the correct CPU, you can skip the THREAD_CAN_MIGRATE check and the call to mi_switch. --- Harrison
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