From mboxrd@z Thu Jan 1 00:00:00 1970 From: Shan Wei Subject: Re: [PATCH 4/9] net: openvswitch: use this_cpu_ptr per-cpu helper Date: Thu, 01 Nov 2012 18:07:03 +0800 Message-ID: <509249C7.3080000@gmail.com> References: <50910A04.5000003@gmail.com> <0000013ab7e86f8a-4adb8b81-19be-4264-96f1-924aaf3819f2-000000@email.amazonses.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Cc: dev-yBygre7rU0TnMu66kgdUjQ@public.gmane.org, NetDev , Kernel-Maillist , David Miller To: Christoph Lameter Return-path: In-Reply-To: <0000013ab7e86f8a-4adb8b81-19be-4264-96f1-924aaf3819f2-000000-p/GC64/jrecnJqMo6gzdpkEOCMrvLtNR@public.gmane.org> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: dev-bounces-yBygre7rU0TnMu66kgdUjQ@public.gmane.org Errors-To: dev-bounces-yBygre7rU0TnMu66kgdUjQ@public.gmane.org List-Id: netdev.vger.kernel.org Christoph Lameter said, at 2012/11/1 1:39: > On Wed, 31 Oct 2012, Shan Wei wrote: > >> --- a/net/openvswitch/datapath.c >> +++ b/net/openvswitch/datapath.c >> @@ -208,7 +208,7 @@ void ovs_dp_process_received_packet(struct vport *p, struct sk_buff *skb) >> int error; >> int key_len; >> >> - stats = per_cpu_ptr(dp->stats_percpu, smp_processor_id()); >> + stats = this_cpu_ptr(dp->stats_percpu); > > Well this is an improvement and may be ok if the preemption is disabled at > this point. There is another possibility here to use this_cpu_read/add/inc > instead of determining the pointer to the local cpu first and then > performing operations on the fields. The pointer relocation with > this_cpu_xxx ops is implicit in the instructions and safe against changing > of processors. It would also save us the determination of a pointer to the > current cpus stats structure. yes, this_cpu_ptr just locate the point to current cpu per-cpu data domain. and then operating [read/write/inc/sub] fields of this per-cpu variable maybe on other cpu because task is rescheduled for preemption, interrupt. But for different field in same per-cpu variable, how to guarantee n_missed and n_hit are from same cpu? this_cpu_read(dp->stats_percpu->n_missed); [processor changed] this_cpu_read(dp->stats_percpu->n_hit); In addition, following usage of per_cpu_ptr can be replaced by this_cpu_read. cpu=get_cpu() .... *per_cpu_ptr(p,cpu) .... .... put_cpu()