Messages in this thread | | | Date | Wed, 12 Apr 2023 12:34:13 +0530 | From | Kautuk Consul <> | Subject | Re: [PATCH] KVM: PPC: BOOK3S: book3s_hv_nested.c: improve branch prediction for k.alloc |
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Hi,
On 2023-04-11 16:35:10, Michael Ellerman wrote: > Kautuk Consul <kconsul@linux.vnet.ibm.com> writes: > > On 2023-04-07 09:01:29, Sean Christopherson wrote: > >> On Fri, Apr 07, 2023, Bagas Sanjaya wrote: > >> > On Fri, Apr 07, 2023 at 05:31:47AM -0400, Kautuk Consul wrote: > >> > > I used the unlikely() macro on the return values of the k.alloc > >> > > calls and found that it changes the code generation a bit. > >> > > Optimize all return paths of k.alloc calls by improving > >> > > branch prediction on return value of k.alloc. > >> > >> Nit, this is improving code generation, not branch prediction. > > Sorry my mistake. > >> > >> > What about below? > >> > > >> > "Improve branch prediction on kmalloc() and kzalloc() call by using > >> > unlikely() macro to optimize their return paths." > >> > >> Another nit, using unlikely() doesn't necessarily provide a measurable optimization. > >> As above, it does often improve code generation for the happy path, but that doesn't > >> always equate to improved performance, e.g. if the CPU can easily predict the branch > >> and/or there is no impact on the cache footprint. > > > I see. I will submit a v2 of the patch with a better and more accurate > > description. Does anyone else have any comments before I do so ? > > In general I think unlikely should be saved for cases where either the > compiler is generating terrible code, or the likelyness of the condition > might be surprising to a human reader. > > eg. if you had some code that does a NULL check and it's *expected* that > the value is NULL, then wrapping that check in likely() actually adds > information for a human reader. > > Also please don't use unlikely in init paths or other cold paths, it > clutters the code (only slightly but a little) and that's not worth the > possible tiny benefit for code that only runs once or infrequently. > > I would expect the compilers to do the right thing in all > these cases without the unlikely. But if you can demonstrate that they > meaningfully improve the code generation with a before/after > dissassembly then I'd be interested. Just FYI, the last email by kautuk.consul.80@gmail.com was by me. That last email contains a diff file attachment which compares 2 files: before my changes and after my changes. This diff file shows a lot of changes in code generation. Im assuming all those changes are made by the compiler towards optimizing all return paths to k.alloc calls. Kindly review and comment. > cheers
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