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
