Messages in this thread | | | Subject | Re: [PATCH v3 5/6] powerpc/pseries: implement paravirt qspinlocks for SPLPAR | From | Waiman Long <> | Date | Thu, 23 Jul 2020 17:58:43 -0400 |
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On 7/23/20 3:58 PM, peterz@infradead.org wrote: > On Thu, Jul 23, 2020 at 03:04:13PM -0400, Waiman Long wrote: >> On 7/23/20 2:47 PM, peterz@infradead.org wrote: >>> On Thu, Jul 23, 2020 at 02:32:36PM -0400, Waiman Long wrote: >>>> BTW, do you have any comment on my v2 lock holder cpu info qspinlock patch? >>>> I will have to update the patch to fix the reported 0-day test problem, but >>>> I want to collect other feedback before sending out v3. >>> I want to say I hate it all, it adds instructions to a path we spend an >>> aweful lot of time optimizing without really getting anything back for >>> it. >> It does add some extra instruction that may slow it down slightly, but I >> don't agree that it gives nothing back. The cpu lock holder information can >> be useful in analyzing crash dumps and in some debugging situation. I think >> it can be useful in RHEL for this readon. How about an x86 config option to >> allow distros to decide if they want to have it enabled? I will make sure >> that it will have no performance degradation if the option is not enabled. > Config knobs suck too; they create a maintenance burden (we get to make > sure all the permutations works/build/etc..) and effectively nobody uses > them, since world+dog uses what distros pick. > > Anyway, instead of adding a second per-cpu variable, can you see how > horrible something like this is: > > unsigned char adds(unsigned char var, unsigned char val) > { > unsigned short sat = 0xff, tmp = var; > > asm ("addb %[val], %b[var];" > "cmovc %[sat], %[var];" > : [var] "+r" (tmp) > : [val] "ir" (val), [sat] "r" (sat) > ); > > return tmp; > } > > Another thing to try is, instead of threading that lockval throughout > the thing, simply: > > #define _Q_LOCKED_VAL this_cpu_read_stable(cpu_sat) > > or combined with the above > > #define _Q_LOCKED_VAL adds(this_cpu_read_stable(cpu_number), 2) > > and see if the compiler really makes a mess of things. > Thanks for the suggestion. I will try that out.
Cheers, Longman
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