[PATCH v2 4/4] net: apply __GFP_NO_AUTOINIT to AF_UNIX sk_buff allocations

[Alexander Potapenko]

On Fri, May 17, 2019 at 2:26 AM Kees Cook <keescook at chromium.org> wrote:
>
> On Thu, May 16, 2019 at 09:53:01AM -0700, Kees Cook wrote:
> > On Tue, May 14, 2019 at 04:35:37PM +0200, Alexander Potapenko wrote:
> > > Add sock_alloc_send_pskb_noinit(), which is similar to
> > > sock_alloc_send_pskb(), but allocates with __GFP_NO_AUTOINIT.
> > > This helps reduce the slowdown on hackbench in the init_on_alloc mode
> > > from 6.84% to 3.45%.
> >
> > Out of curiosity, why the creation of the new function over adding a
> > gfp flag argument to sock_alloc_send_pskb() and updating callers? (There
> > are only 6 callers, and this change already updates 2 of those.)
> >
> > > Slowdown for the initialization features compared to init_on_free=0,
> > > init_on_alloc=0:
> > >
> > > hackbench, init_on_free=1:  +7.71% sys time (st.err 0.45%)
> > > hackbench, init_on_alloc=1: +3.45% sys time (st.err 0.86%)
>
> So I've run some of my own wall-clock timings of kernel builds (which
> should be an pretty big "worst case" situation, and I see much smaller
> performance changes:
How many cores were you using? I suspect the numbers may vary a bit
depending on that.
> everything off
>         Run times: 289.18 288.61 289.66 287.71 287.67
>         Min: 287.67 Max: 289.66 Mean: 288.57 Std Dev: 0.79
>                 baseline
>
> init_on_alloc=1
>         Run times: 289.72 286.95 287.87 287.34 287.35
>         Min: 286.95 Max: 289.72 Mean: 287.85 Std Dev: 0.98
>                 0.25% faster (within the std dev noise)
>
> init_on_free=1
>         Run times: 303.26 301.44 301.19 301.55 301.39
>         Min: 301.19 Max: 303.26 Mean: 301.77 Std Dev: 0.75
>                 4.57% slower
>
> init_on_free=1 with the PAX_MEMORY_SANITIZE slabs excluded:
>         Run times: 299.19 299.85 298.95 298.23 298.64
>         Min: 298.23 Max: 299.85 Mean: 298.97 Std Dev: 0.55
>                 3.60% slower
>
> So the tuning certainly improved things by 1%. My perf numbers don't
> show the 24% hit you were seeing at all, though.
Note that 24% is the _sys_ time slowdown. The wall time slowdown seen
in this case was 8.34%

> > In the commit log it might be worth mentioning that this is only
> > changing the init_on_alloc case (in case it's not already obvious to
> > folks). Perhaps there needs to be a split of __GFP_NO_AUTOINIT into
> > __GFP_NO_AUTO_ALLOC_INIT and __GFP_NO_AUTO_FREE_INIT? Right now
> > __GFP_NO_AUTOINIT is only checked for init_on_alloc:
>
> I was obviously crazy here. :) GFP isn't present for free(), but a SLAB
> flag works (as was done in PAX_MEMORY_SANITIZE). I'll send the patch I
> used for the above timing test.
>
> --
> Kees Cook



-- 
Alexander Potapenko
Software Engineer

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