Re: [PATCH v3 1/3] RISC-V: Issue a local tlbflush if possible.
On Thu, 2019-08-22 at 06:27 +0200, h...@lst.de wrote: > On Thu, Aug 22, 2019 at 04:01:24AM +, Atish Patra wrote: > > The downside of this is that for every !cmask case in true SMP > > (more > > common probably) it will execute 2 extra cpumask instructions. As > > tlbflush path is in performance critical path, I think we should > > favor > > more common case (SMP with more than 1 core). > > Actually, looking at both the current mainline code, and the code > from my > cleanups tree I don't think remote_sfence_vma / __sbi_tlb_flush_range > can ever be called with NULL cpumask, as we always have a valid mm. > Yes. You are correct. As both cpumask functions here will crash if cpumask is null, we should probably leave a harmless comment to warn the consequeunce of cpumask being null. > So this is a bit of a moot point, and we can drop andling that case > entirely. With that we can also use a simple if / else for the local > cpu only vs remote case. Done. > Btw, what was the reason you didn't like > using cpumask_any_but like x86, which should be more efficient than > cpumask_test_cpu + hweigt? I had it in v2 patch but removed as it can potentially return garbage value if cpumask is empty. However, we are already checking empty cpumask before the local cpu check. I will replace cpumask_test_cpu + hweight with cpumask_any_but(). -- Regards, Atish
Re: [PATCH v3 1/3] RISC-V: Issue a local tlbflush if possible.
On Thu, Aug 22, 2019 at 04:01:24AM +, Atish Patra wrote: > The downside of this is that for every !cmask case in true SMP (more > common probably) it will execute 2 extra cpumask instructions. As > tlbflush path is in performance critical path, I think we should favor > more common case (SMP with more than 1 core). Actually, looking at both the current mainline code, and the code from my cleanups tree I don't think remote_sfence_vma / __sbi_tlb_flush_range can ever be called with NULL cpumask, as we always have a valid mm. So this is a bit of a moot point, and we can drop andling that case entirely. With that we can also use a simple if / else for the local cpu only vs remote case. Btw, what was the reason you didn't like using cpumask_any_but like x86, which should be more efficient than cpumask_test_cpu + hweigt?
Re: [PATCH v3 1/3] RISC-V: Issue a local tlbflush if possible.
On Thu, 2019-08-22 at 03:46 +0200, Christoph Hellwig wrote:
> On Wed, Aug 21, 2019 at 05:46:42PM -0700, Atish Patra wrote:
> > In RISC-V, tlb flush happens via SBI which is expensive. If the
> > local
> > cpu is the only cpu in cpumask, there is no need to invoke a SBI
> > call.
> >
> > Just do a local flush and return.
> >
> > Signed-off-by: Atish Patra
> > ---
> > arch/riscv/mm/tlbflush.c | 15 +++
> > 1 file changed, 15 insertions(+)
> >
> > diff --git a/arch/riscv/mm/tlbflush.c b/arch/riscv/mm/tlbflush.c
> > index df93b26f1b9d..36430ee3bed9 100644
> > --- a/arch/riscv/mm/tlbflush.c
> > +++ b/arch/riscv/mm/tlbflush.c
> > @@ -2,6 +2,7 @@
> >
> > #include
> > #include
> > +#include
> > #include
> >
> > void flush_tlb_all(void)
> > @@ -13,9 +14,23 @@ static void __sbi_tlb_flush_range(struct cpumask
> > *cmask, unsigned long start,
> > unsigned long size)
> > {
> > struct cpumask hmask;
> > + unsigned int cpuid = get_cpu();
> >
> > + if (!cmask) {
> > + riscv_cpuid_to_hartid_mask(cpu_online_mask, );
> > + goto issue_sfence;
> > + }
> > +
> > + if (cpumask_test_cpu(cpuid, cmask) && cpumask_weight(cmask) ==
> > 1) {
> > + local_flush_tlb_all();
> > + goto done;
> > + }
>
> I think a single core on a SMP kernel is a valid enough use case
> given
> how litte distros still have UP kernels. So Maybe this shiuld rather
> be:
>
> if (!cmask)
> cmask = cpu_online_mask;
>
> if (cpumask_test_cpu(cpuid, cmask) && cpumask_weight(cmask) ==
> 1) {
> local_flush_tlb_all();
> } else {
> riscv_cpuid_to_hartid_mask(cmask, );
> sbi_remote_sfence_vma(hmask.bits, start, size);
> }
The downside of this is that for every !cmask case in true SMP (more
common probably) it will execute 2 extra cpumask instructions. As
tlbflush path is in performance critical path, I think we should favor
more common case (SMP with more than 1 core).
Thoughts ?
--
Regards,
Atish
Re: [PATCH v3 1/3] RISC-V: Issue a local tlbflush if possible.
On Wed, Aug 21, 2019 at 05:46:42PM -0700, Atish Patra wrote:
> In RISC-V, tlb flush happens via SBI which is expensive. If the local
> cpu is the only cpu in cpumask, there is no need to invoke a SBI call.
>
> Just do a local flush and return.
>
> Signed-off-by: Atish Patra
> ---
> arch/riscv/mm/tlbflush.c | 15 +++
> 1 file changed, 15 insertions(+)
>
> diff --git a/arch/riscv/mm/tlbflush.c b/arch/riscv/mm/tlbflush.c
> index df93b26f1b9d..36430ee3bed9 100644
> --- a/arch/riscv/mm/tlbflush.c
> +++ b/arch/riscv/mm/tlbflush.c
> @@ -2,6 +2,7 @@
>
> #include
> #include
> +#include
> #include
>
> void flush_tlb_all(void)
> @@ -13,9 +14,23 @@ static void __sbi_tlb_flush_range(struct cpumask *cmask,
> unsigned long start,
> unsigned long size)
> {
> struct cpumask hmask;
> + unsigned int cpuid = get_cpu();
>
> + if (!cmask) {
> + riscv_cpuid_to_hartid_mask(cpu_online_mask, );
> + goto issue_sfence;
> + }
> +
> + if (cpumask_test_cpu(cpuid, cmask) && cpumask_weight(cmask) == 1) {
> + local_flush_tlb_all();
> + goto done;
> + }
I think a single core on a SMP kernel is a valid enough use case given
how litte distros still have UP kernels. So Maybe this shiuld rather be:
if (!cmask)
cmask = cpu_online_mask;
if (cpumask_test_cpu(cpuid, cmask) && cpumask_weight(cmask) == 1) {
local_flush_tlb_all();
} else {
riscv_cpuid_to_hartid_mask(cmask, );
sbi_remote_sfence_vma(hmask.bits, start, size);
}
[PATCH v3 1/3] RISC-V: Issue a local tlbflush if possible.
In RISC-V, tlb flush happens via SBI which is expensive. If the local
cpu is the only cpu in cpumask, there is no need to invoke a SBI call.
Just do a local flush and return.
Signed-off-by: Atish Patra
---
arch/riscv/mm/tlbflush.c | 15 +++
1 file changed, 15 insertions(+)
diff --git a/arch/riscv/mm/tlbflush.c b/arch/riscv/mm/tlbflush.c
index df93b26f1b9d..36430ee3bed9 100644
--- a/arch/riscv/mm/tlbflush.c
+++ b/arch/riscv/mm/tlbflush.c
@@ -2,6 +2,7 @@
#include
#include
+#include
#include
void flush_tlb_all(void)
@@ -13,9 +14,23 @@ static void __sbi_tlb_flush_range(struct cpumask *cmask,
unsigned long start,
unsigned long size)
{
struct cpumask hmask;
+ unsigned int cpuid = get_cpu();
+ if (!cmask) {
+ riscv_cpuid_to_hartid_mask(cpu_online_mask, );
+ goto issue_sfence;
+ }
+
+ if (cpumask_test_cpu(cpuid, cmask) && cpumask_weight(cmask) == 1) {
+ local_flush_tlb_all();
+ goto done;
+ }
riscv_cpuid_to_hartid_mask(cmask, );
+
+issue_sfence:
sbi_remote_sfence_vma(hmask.bits, start, size);
+done:
+ put_cpu();
}
void flush_tlb_mm(struct mm_struct *mm)
--
2.21.0
