Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Thu, 25 Oct 2018 02:31:30 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Andi Kleen <ak@linux.intel.com>
Cc:     "Liang, Kan" <kan.liang@linux.intel.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@kernel.org>, linux-kernel@vger.kernel.org,
        bp@alien8.de, Stephane Eranian <eranian@google.com>
Subject: Re: [PATCH 1/2] perf: Add munmap callback
Message-ID: <20181025003130.GU3109@worktop.c.hoisthospitality.com>
References: <20181024151116.30935-1-kan.liang@linux.intel.com>
 <20181024162334.GG6218@tassilo.jf.intel.com>
 <20181024163208.GC15106@kernel.org>
 <18d5fcf4-84b2-806c-eb2c-787b96e0fb90@linux.intel.com>
 <20181024182854.GH6218@tassilo.jf.intel.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20181024182854.GH6218@tassilo.jf.intel.com>
User-Agent: Mutt/1.5.22.1 (2013-10-16)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Wed, Oct 24, 2018 at 11:28:54AM -0700, Andi Kleen wrote:
> > > void perf_event_mmap(struct vm_area_struct *vma)
> > > {
> > >          struct perf_mmap_event mmap_event;
> > > 
> > >          if (!atomic_read(&nr_mmap_events))
> > >                  return;
> > > <SNIP>
> > > }
> > > 
> > 
> > Thanks. I'll add the nr_mmap_events check in V2.
> 
> No, that's the wrong check here. The PEBS flush is independent of mmap
> events being requested.
> 
> If anything would need to check for any PEBS events active, which
> would need a new counter.  I think the easiest is to just check if 
> this_cpu_ptr(&sched_cb_list)
> is empty, which should be good enough.

It is just the CLI+STI, not PUSHF;CLI+POPF that are required and that is
a lot cheaper already. Also, you need to have preemption disabled in
order to check the per-cpu cb list.

So I don't think it really makes much sense to try and frob a fast path
in there.