linux-kernel - Re: [PATCH] TRACING: FTRACE: Use xarray structure for ftrace syscalls

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Date:   Wed, 13 Nov 2019 14:24:59 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Hassan Naveed <hnaveed@...ecomp.com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Paul Burton <pburton@...ecomp.com>
Subject: Re: [PATCH] TRACING: FTRACE: Use xarray structure for ftrace
 syscalls


ping?

-- Steve


On Tue, 22 Oct 2019 17:45:51 -0400
Steven Rostedt <rostedt@...dmis.org> wrote:

> On Tue, 22 Oct 2019 15:51:04 -0400
> Steven Rostedt <rostedt@...dmis.org> wrote:
> 
> > >  static struct syscall_metadata *syscall_nr_to_meta(int nr)
> > >  {
> > > -	if (!syscalls_metadata || nr >= NR_syscalls || nr < 0)
> > > -		return NULL;
> > > -
> > > -	return syscalls_metadata[nr];
> > > +	return xa_load(&syscalls_metadata, (unsigned long)nr);
> > >  }
> > >    
> 
> There appears to be a slight overhead to this for archs that do not
> have a sparse syscall array. I wonder if we should make this only
> applicable for archs (via adding a HAVE_SPARSE_SYSCALL_NR define and
> checking against it). Then if an arch doesn't have a sparse array of
> system calls, it uses a normal lookup, but for archs that do, it can
> define this for this type of lookup.
> 
> There's not much to this patch, so it wouldn't be too difficult to
> support both methods.
> 
> Without this patch I ran:
> 
> # trace-cmd start -e syscalls
> # /work/c/hackbench 50
> Time: 15.702
> # /work/c/hackbench 50
> Time: 15.932
> # /work/c/hackbench 50
> Time: 15.893
> # /work/c/hackbench 50
> Time: 16.038
> # /work/c/hackbench 50
> Time: 15.429
> 
> 
> With the patch it had:
> 
> # trace-cmd start -e syscalls
> # /work/c/hackbench 50
> Time: 16.582
> # /work/c/hackbench 50
> Time: 15.972
> # /work/c/hackbench 50
> Time: 16.078
> # /work/c/hackbench 50
> Time: 16.133
> # /work/c/hackbench 50
> Time: 16.263
> 
> -- Steve