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

[Kees Cook <keescook@chromium.org>]

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:

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.

> 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