Le Wed, Oct 29, 2025 at 03:13:59PM +0100, Valentin Schneider a écrit : > On 29/10/25 11:31, Frederic Weisbecker wrote: > > Le Wed, Oct 29, 2025 at 11:16:23AM +0100, Valentin Schneider a écrit : > >> On 28/10/25 16:59, Frederic Weisbecker wrote: > >> > Le Fri, Oct 10, 2025 at 05:38:37PM +0200, Valentin Schneider a écrit : > >> >> @@ -171,8 +172,27 @@ For 32-bit we have the following conventions - > >> >> kernel is built with > >> >> andq $(~PTI_USER_PGTABLE_AND_PCID_MASK), \reg > >> >> .endm > >> >> > >> >> -.macro COALESCE_TLBI > >> >> +.macro COALESCE_TLBI scratch_reg:req > >> >> #ifdef CONFIG_COALESCE_TLBI > >> >> + /* No point in doing this for housekeeping CPUs */ > >> >> + movslq PER_CPU_VAR(cpu_number), \scratch_reg > >> >> + bt \scratch_reg, tick_nohz_full_mask(%rip) > >> >> + jnc .Lend_tlbi_\@ > >> > > >> > I assume it's not possible to have a static call/branch to > >> > take care of all this ? > >> > > >> > >> I think technically yes, but that would have to be a per-cpu patchable > >> location, which would mean something like each CPU having its own copy of > >> that text page... Unless there's some existing way to statically optimize > >> > >> if (cpumask_test_cpu(smp_processor_id(), mask)) > >> > >> where @mask is a boot-time constant (i.e. the nohz_full mask). > > > > Or just check housekeeping_overriden static key before everything. This one > > is > > enabled only if either nohz_full, isolcpus or cpuset isolated partition > > (well, > > it's on the way for the last one) are running, but those are all niche, > > which > > means you spare 99.999% kernel usecases. > > > > Oh right, if NOHZ_FULL is actually in use. > > Yeah that housekeeping key could do since, at least for the cmdline > approach, it's set during start_kernel(). I need to have a think about the > runtime cpuset case.
You can ignore the runtime thing and simply check the static key before reading the housekeeping mask. For now nohz_full is only enabled by cmdline. > Given we have ALTERNATIVE's in there I assume something like a > boot-time-driven static key could do, but I haven't found out yet if and > how that can be shoved in an ASM file. Right, I thought I had seen static keys in ASM already but I can't find it anymore. arch/x86/include/asm/jump_label.h is full of reusable magic though. Thanks. -- Frederic Weisbecker SUSE Labs
