From mboxrd@z Thu Jan 1 00:00:00 1970 From: Nick Piggin Date: Sun, 16 Apr 2006 07:06:14 +0000 Subject: Re: [PATCH 00/05] robust per_cpu allocation for modules Message-Id: <4441ECE6.5010709@yahoo.com.au> List-Id: References: <1145049535.1336.128.camel@localhost.localdomain> <4440855A.7040203@yahoo.com.au> <4441B02D.4000405@yahoo.com.au> In-Reply-To: MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: Steven Rostedt Cc: LKML , Andrew Morton , Linus Torvalds , Ingo Molnar , Thomas Gleixner , Andi Kleen , Martin Mares , bjornw@axis.com, schwidefsky@de.ibm.com, benedict.gaster@superh.com, lethal@linux-sh.org, Chris Zankel , Marc Gauthier , Joe Taylor , David Mosberger-Tang , rth@twiddle.net, spyro@f2s.com, starvik@axis.com, tony.luck@intel.com, linux-ia64@vger.kernel.org, ralf@linux-mips.org, linux-mips@linux-mips.org, grundler@parisc-linux.org, parisc-linux@parisc-linux.org, linuxppc-dev@ozlabs.org, paulus@samba.org, linux390@de.ibm.com, davem@davemloft.net Steven Rostedt wrote: > On Sun, 16 Apr 2006, Nick Piggin wrote: >>Why is your module using so much per-cpu memory, anyway? > > > Wasn't my module anyway. The problem appeared in the -rt patch set, when > tracing was turned on. Some module was affected, and grew it's per_cpu > size by quite a bit. In fact we had to increase PERCPU_ENOUGH_ROOM by up > to something like 300K. Well that's easy then, just configure PERCPU_ENOUGH_ROOM to be larger when tracing is on in the -rt patchset? Or use alloc_percpu for the tracing data? >>I don't think it would have been hard for the original author to make >>it robust... just not both fast and robust. PERCPU_ENOUGH_ROOM seems >>like an ugly hack at first glance, but I'm fairly sure it was a result >>of design choices. > > Yeah, and I discovered the reasons for those choices as I worked on this. > I've put a little more thought into this and still think there's a > solution to not slow things down. > > Since the per_cpu_offset section is still smaller than the > PERCPU_ENOUGH_ROOM and robust, I could still copy it into a per cpu memory > field, and even add the __per_cpu_offset to it. This would still save > quite a bit of space. Well I don't think making it per-cpu would help much (presumably it is not going to be written to very frequently) -- I guess it would be a small advantage on NUMA. The main problem is the extra load in the fastpath. You can't start the next load until the results of the first come back. > So now I'm asking for advice on some ideas that can be a work around to > keep the robustness and speed. > > Is there a way (for archs that support it) to allocate memory in a per cpu > manner. So each CPU would have its own variable table in the memory that > is best of it. Then have a field (like the pda in x86_64) to point to > this section, and use the linker offsets to index and find the per_cpu > variables. > > So this solution still has one more redirection than the current solution > (per_cpu_offset__##var -> __per_cpu_offset -> actual_var where as the > current solution is __per_cpu_offset -> actual_var), but all the loads > would be done from memory that would only be specified for a particular > CPU. > > The generic case would still be the same as the patches I already sent, > but the archs that can support it, can have something like the above. > > Would something like that be acceptible? I still don't understand what the justification is for slowing down this critical bit of infrastructure for something that is only a problem in the -rt patchset, and even then only a problem when tracing is enabled. -- SUSE Labs, Novell Inc. Send instant messages to your online friends http://au.messenger.yahoo.com