From mboxrd@z Thu Jan 1 00:00:00 1970 From: Brent Casavant Date: Tue, 24 Jan 2006 22:12:09 +0000 Subject: RE: [PATCH] SN2 user-MMIO CPU migration Message-Id: <20060124160438.S90635@chenjesu.americas.sgi.com> List-Id: References: <20060118163305.Y42462@chenjesu.americas.sgi.com> In-Reply-To: <20060118163305.Y42462@chenjesu.americas.sgi.com> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: linux-ia64@vger.kernel.org On Tue, 24 Jan 2006, Chen, Kenneth W wrote: > Yeah, I'm not thrilled either. Currently, on context switch kernel > doesn't read thread_info.cpu, nor any other neighboring field. With > the patch, it needs to do a cacheline read and possibly an update. > If unlucky, it might be two cache lines with last_cpu 92 bytes away > from thread_info.cpu. Perhaps, last_cpu should be next to cpu field. Done. > On the other hand, it may not be too bad because on kernel exit, flags > field in thread_info is accessed for TIF_* and if kernel exit path is > taken back-to-back with context switch, the net effect maybe small. Actually, IA64_HAS_EXTRA_STATE() reads the TIF_* flags, and is called in __switch_to(), which is in very close proximity to checking last_cpu. I don't think there'll be a problem here, particularly as I've now moved last_cpu to neighbor cpu. Updated patch following in a few minutes. Brent -- Brent Casavant All music is folk music. I ain't bcasavan@sgi.com never heard a horse sing a song. Silicon Graphics, Inc. -- Louis Armstrong