| Message ID | 20240503171847.68267-2-puranjay@kernel.org (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | [v3,1/2] arm64/arch_timer: include <linux/percpu.h> | expand |
Quoting Puranjay Mohan (2024-05-03 10:18:47) > Remove the percpu variable cpu_number as it is used only in > set_smp_ipi_range() as a dummy variable to be passed to ipi_handler(). > Use irq_stat in place of cpu_number here like arm32. > > [1] https://github.com/puranjaymohan/linux/commit/77d3fdd Maybe you should have put this under the cut for the patch so its on the mailing list, and lore. > > Signed-off-by: Puranjay Mohan <puranjay@kernel.org> > Acked-by: Mark Rutland <mark.rutland@arm.com> > --- Reviewed-by: Stephen Boyd <swboyd@chromium.org> > diff --git a/arch/arm64/kernel/smp.c b/arch/arm64/kernel/smp.c > index 4ced34f62dab..98d4e352c3d0 100644 > --- a/arch/arm64/kernel/smp.c > +++ b/arch/arm64/kernel/smp.c > @@ -1021,12 +1016,12 @@ void __init set_smp_ipi_range(int ipi_base, int n) > > if (ipi_should_be_nmi(i)) { > err = request_percpu_nmi(ipi_base + i, ipi_handler, > - "IPI", &cpu_number); > + "IPI", &irq_stat); I know this is mirroring arm, but it would be nice to have a comment saying that we need some percpu variable here even if the handler doesn't use it.
diff --git a/arch/arm64/include/asm/smp.h b/arch/arm64/include/asm/smp.h index efb13112b408..2510eec026f7 100644 --- a/arch/arm64/include/asm/smp.h +++ b/arch/arm64/include/asm/smp.h @@ -25,22 +25,11 @@ #ifndef __ASSEMBLY__ -#include <asm/percpu.h> - #include <linux/threads.h> #include <linux/cpumask.h> #include <linux/thread_info.h> -DECLARE_PER_CPU_READ_MOSTLY(int, cpu_number); - -/* - * We don't use this_cpu_read(cpu_number) as that has implicit writes to - * preempt_count, and associated (compiler) barriers, that we'd like to avoid - * the expense of. If we're preemptible, the value can be stale at use anyway. - * And we can't use this_cpu_ptr() either, as that winds up recursing back - * here under CONFIG_DEBUG_PREEMPT=y. - */ -#define raw_smp_processor_id() (*raw_cpu_ptr(&cpu_number)) +#define raw_smp_processor_id() (current_thread_info()->cpu) /* * Logical CPU mapping. diff --git a/arch/arm64/kernel/smp.c b/arch/arm64/kernel/smp.c index 4ced34f62dab..98d4e352c3d0 100644 --- a/arch/arm64/kernel/smp.c +++ b/arch/arm64/kernel/smp.c @@ -55,9 +55,6 @@ #include <trace/events/ipi.h> -DEFINE_PER_CPU_READ_MOSTLY(int, cpu_number); -EXPORT_PER_CPU_SYMBOL(cpu_number); - /* * as from 2.5, kernels no longer have an init_tasks structure * so we need some other way of telling a new secondary core @@ -742,8 +739,6 @@ void __init smp_prepare_cpus(unsigned int max_cpus) */ for_each_possible_cpu(cpu) { - per_cpu(cpu_number, cpu) = cpu; - if (cpu == smp_processor_id()) continue; @@ -1021,12 +1016,12 @@ void __init set_smp_ipi_range(int ipi_base, int n) if (ipi_should_be_nmi(i)) { err = request_percpu_nmi(ipi_base + i, ipi_handler, - "IPI", &cpu_number); + "IPI", &irq_stat); WARN(err, "Could not request IPI %d as NMI, err=%d\n", i, err); } else { err = request_percpu_irq(ipi_base + i, ipi_handler, - "IPI", &cpu_number); + "IPI", &irq_stat); WARN(err, "Could not request IPI %d as IRQ, err=%d\n", i, err); }
Historically, arm64 implemented raw_smp_processor_id() as a read of current_thread_info()->cpu. This changed when arm64 moved thread_info into task struct, as at the time CONFIG_THREAD_INFO_IN_TASK made core code use thread_struct::cpu for the cpu number, and due to header dependencies prevented using this in raw_smp_processor_id(). As a workaround, we moved to using a percpu variable in commit: 57c82954e77fa12c ("arm64: make cpu number a percpu variable") Since then, thread_info::cpu was reintroduced, and core code was made to use this in commits: 001430c1910df65a ("arm64: add CPU field to struct thread_info") bcf9033e5449bdca ("sched: move CPU field back into thread_info if THREAD_INFO_IN_TASK=y") Consequently it is possible to use current_thread_info()->cpu again. This decreases the number of emitted instructions like in the following example: Dump of assembler code for function bpf_get_smp_processor_id: 0xffff8000802cd608 <+0>: nop 0xffff8000802cd60c <+4>: nop 0xffff8000802cd610 <+8>: adrp x0, 0xffff800082138000 0xffff8000802cd614 <+12>: mrs x1, tpidr_el1 0xffff8000802cd618 <+16>: add x0, x0, #0x8 0xffff8000802cd61c <+20>: ldrsw x0, [x0, x1] 0xffff8000802cd620 <+24>: ret After this patch: Dump of assembler code for function bpf_get_smp_processor_id: 0xffff8000802c9130 <+0>: nop 0xffff8000802c9134 <+4>: nop 0xffff8000802c9138 <+8>: mrs x0, sp_el0 0xffff8000802c913c <+12>: ldr w0, [x0, #24] 0xffff8000802c9140 <+16>: ret A microbenchmark[1] was built to measure the performance improvement provided by this change. It calls the following function given number of times and finds the runtime overhead: static noinline int get_cpu_id(void) { return smp_processor_id(); } Run the benchmark like: modprobe smp_processor_id nr_function_calls=1000000000 +--------------------------+------------------------+ | | Number of Calls | Time taken | +--------+-----------------+------------------------+ | Before | 1000000000 | 1602888401ns | +--------+-----------------+------------------------+ | After | 1000000000 | 1206212658ns | +--------+-----------------+------------------------+ | Difference (decrease) | 396675743ns (24.74%) | +---------------------------------------------------+ Remove the percpu variable cpu_number as it is used only in set_smp_ipi_range() as a dummy variable to be passed to ipi_handler(). Use irq_stat in place of cpu_number here like arm32. [1] https://github.com/puranjaymohan/linux/commit/77d3fdd Signed-off-by: Puranjay Mohan <puranjay@kernel.org> Acked-by: Mark Rutland <mark.rutland@arm.com> --- arch/arm64/include/asm/smp.h | 13 +------------ arch/arm64/kernel/smp.c | 9 ++------- 2 files changed, 3 insertions(+), 19 deletions(-)