Re: [PATCH v5 03/13] riscv: Use IPIs for remote cache/TLB flushes by default

From: Anup Patel
Date: Mon Mar 11 2024 - 00:12:56 EST

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On Mon, Mar 11, 2024 at 9:34 AM Anup Patel <apatel@xxxxxxxxxxxxxxxx> wrote:
>
> On Mon, Mar 11, 2024 at 8:37 AM Stefan O'Rear <sorear@xxxxxxxxxxxx> wrote:
> >
> > On Thu, Feb 29, 2024, at 6:21 PM, Samuel Holland wrote:
> > > An IPI backend is always required in an SMP configuration, but an SBI
> > > implementation is not. For example, SBI will be unavailable when the
> > > kernel runs in M mode.
> > >
> > > Generally, IPIs are assumed to be faster than SBI calls due to the SBI
> > > context switch overhead. However, when SBI is used as the IPI backend,
> > > then the context switch cost must be paid anyway, and performing the
> > > cache/TLB flush directly in the SBI implementation is more efficient
> > > than inserting an interrupt to the kernel. This is the only scenario
> > > where riscv_ipi_set_virq_range()'s use_for_rfence parameter is false.
> > >
> > > Thus, it makes sense for remote fences to use IPIs by default, and make
> > > the SBI remote fence extension the special case.
> >
> > The historical intention of providing SBI calls for remote fences is that
> > it abstracts over hardware that allows for performing remote fences
> > _without an IPI at all_. The TH1520 is an example of such hardware, since
> > it can (at least according to the documentation) perform broadcast fence,
> > fence.i, and sfence.vma operations using bits in the mhint register.
> >
> > T-Head's public opensbi repository doesn't actually use this feature, and
> > in general SBI remote fences come from a much more optimistic time about
> > how much we can successfully hide from supervisor software. But I don't
> > think we can generalize that an IPI will always do less work than a SBI
> > remote fence.
>
> On platforms where SBI is the only way of injecting IPIs in S-mode, the
> SBI based remote fences are actually much faster. This is because on
> such platforms injecting an IPI from a HART to a set of HARTs will
> require multiple SBI calls which can be directly replaced by one (or
> few) SBI remote fence SBI calls.
>
> Most of the current platforms still have M-mode CLINT and depend on
> SBI IPI for S-mode IPI injection so we should not slow down remote
> fences for these platforms.
>
> Until all platforms have moved to RISC-V AIA (which supports S-mode
> IPIs), we should keep this boot-time choice of SBI RFENCEs versus
> direct S-mode IPIs.
>
> IMO, this patch is ahead of its time.

I think this patch is fine since it replaces the static key
riscv_ipi_for_rfence with riscv_sbi_for_rfence.

I got confused by the removal of riscv_ipi_for_rfence.

Reviewed-by: Anup Patel <anup@xxxxxxxxxxxxxx>

Regards,
Anup

>
> Regards,
> Anup
>
> >
> > -s
> >
> > > sbi_ipi_init() already checks riscv_ipi_have_virq_range(), so it only
> > > calls riscv_ipi_set_virq_range() when no other IPI device is available.
> > > So we can move the static key and drop the use_for_rfence parameter.
> > >
> > > Furthermore, the static branch only makes sense when CONFIG_RISCV_SBI is
> > > enabled. Optherwise, IPIs must be used. Add a fallback definition of
> > > riscv_use_sbi_for_rfence() which handles this case and removes the need
> > > to check CONFIG_RISCV_SBI elsewhere, such as in cacheflush.c.
> > >
> > > Signed-off-by: Samuel Holland <samuel.holland@xxxxxxxxxx>
> > > ---
> > >
> > > Changes in v5:
> > > - Also switch to riscv_use_sbi_for_rfence() in asm/pgalloc.h
> > >
> > > Changes in v4:
> > > - New patch for v4
> > >
> > > arch/riscv/include/asm/pgalloc.h | 7 ++++---
> > > arch/riscv/include/asm/sbi.h | 4 ++++
> > > arch/riscv/include/asm/smp.h | 15 ++-------------
> > > arch/riscv/kernel/sbi-ipi.c | 11 ++++++++++-
> > > arch/riscv/kernel/smp.c | 11 +----------
> > > arch/riscv/mm/cacheflush.c | 5 ++---
> > > arch/riscv/mm/tlbflush.c | 31 ++++++++++++++-----------------
> > > drivers/clocksource/timer-clint.c | 2 +-
> > > 8 files changed, 38 insertions(+), 48 deletions(-)
> > >
> > > diff --git a/arch/riscv/include/asm/pgalloc.h b/arch/riscv/include/asm/pgalloc.h
> > > index 87468f67951a..6578054977ef 100644
> > > --- a/arch/riscv/include/asm/pgalloc.h
> > > +++ b/arch/riscv/include/asm/pgalloc.h
> > > @@ -8,6 +8,7 @@
> > > #define _ASM_RISCV_PGALLOC_H
> > >
> > > #include <linux/mm.h>
> > > +#include <asm/sbi.h>
> > > #include <asm/tlb.h>
> > >
> > > #ifdef CONFIG_MMU
> > > @@ -90,10 +91,10 @@ static inline pud_t *pud_alloc_one(struct mm_struct
> > > *mm, unsigned long addr)
> > >
> > > static inline void riscv_tlb_remove_ptdesc(struct mmu_gather *tlb, void *pt)
> > > {
> > > - if (riscv_use_ipi_for_rfence())
> > > - tlb_remove_page_ptdesc(tlb, pt);
> > > - else
> > > + if (riscv_use_sbi_for_rfence())
> > > tlb_remove_ptdesc(tlb, pt);
> > > + else
> > > + tlb_remove_page_ptdesc(tlb, pt);
> > > }
> > >
> > > #define pud_free pud_free
> > > diff --git a/arch/riscv/include/asm/sbi.h b/arch/riscv/include/asm/sbi.h
> > > index 6e68f8dff76b..ea84392ca9d7 100644
> > > --- a/arch/riscv/include/asm/sbi.h
> > > +++ b/arch/riscv/include/asm/sbi.h
> > > @@ -375,8 +375,12 @@ unsigned long riscv_cached_marchid(unsigned int cpu_id);
> > > unsigned long riscv_cached_mimpid(unsigned int cpu_id);
> > >
> > > #if IS_ENABLED(CONFIG_SMP) && IS_ENABLED(CONFIG_RISCV_SBI)
> > > +DECLARE_STATIC_KEY_FALSE(riscv_sbi_for_rfence);
> > > +#define riscv_use_sbi_for_rfence() \
> > > + static_branch_unlikely(&riscv_sbi_for_rfence)
> > > void sbi_ipi_init(void);
> > > #else
> > > +static inline bool riscv_use_sbi_for_rfence(void) { return false; }
> > > static inline void sbi_ipi_init(void) { }
> > > #endif
> > >
> > > diff --git a/arch/riscv/include/asm/smp.h b/arch/riscv/include/asm/smp.h
> > > index 0d555847cde6..7ac80e9f2288 100644
> > > --- a/arch/riscv/include/asm/smp.h
> > > +++ b/arch/riscv/include/asm/smp.h
> > > @@ -49,12 +49,7 @@ void riscv_ipi_disable(void);
> > > bool riscv_ipi_have_virq_range(void);
> > >
> > > /* Set the IPI interrupt numbers for arch (called by irqchip drivers) */
> > > -void riscv_ipi_set_virq_range(int virq, int nr, bool use_for_rfence);
> > > -
> > > -/* Check if we can use IPIs for remote FENCEs */
> > > -DECLARE_STATIC_KEY_FALSE(riscv_ipi_for_rfence);
> > > -#define riscv_use_ipi_for_rfence() \
> > > - static_branch_unlikely(&riscv_ipi_for_rfence)
> > > +void riscv_ipi_set_virq_range(int virq, int nr);
> > >
> > > /* Check other CPUs stop or not */
> > > bool smp_crash_stop_failed(void);
> > > @@ -104,16 +99,10 @@ static inline bool riscv_ipi_have_virq_range(void)
> > > return false;
> > > }
> > >
> > > -static inline void riscv_ipi_set_virq_range(int virq, int nr,
> > > - bool use_for_rfence)
> > > +static inline void riscv_ipi_set_virq_range(int virq, int nr)
> > > {
> > > }
> > >
> > > -static inline bool riscv_use_ipi_for_rfence(void)
> > > -{
> > > - return false;
> > > -}
> > > -
> > > #endif /* CONFIG_SMP */
> > >
> > > #if defined(CONFIG_HOTPLUG_CPU) && (CONFIG_SMP)
> > > diff --git a/arch/riscv/kernel/sbi-ipi.c b/arch/riscv/kernel/sbi-ipi.c
> > > index a4559695ce62..1026e22955cc 100644
> > > --- a/arch/riscv/kernel/sbi-ipi.c
> > > +++ b/arch/riscv/kernel/sbi-ipi.c
> > > @@ -13,6 +13,9 @@
> > > #include <linux/irqdomain.h>
> > > #include <asm/sbi.h>
> > >
> > > +DEFINE_STATIC_KEY_FALSE(riscv_sbi_for_rfence);
> > > +EXPORT_SYMBOL_GPL(riscv_sbi_for_rfence);
> > > +
> > > static int sbi_ipi_virq;
> > >
> > > static void sbi_ipi_handle(struct irq_desc *desc)
> > > @@ -72,6 +75,12 @@ void __init sbi_ipi_init(void)
> > > "irqchip/sbi-ipi:starting",
> > > sbi_ipi_starting_cpu, NULL);
> > >
> > > - riscv_ipi_set_virq_range(virq, BITS_PER_BYTE, false);
> > > + riscv_ipi_set_virq_range(virq, BITS_PER_BYTE);
> > > pr_info("providing IPIs using SBI IPI extension\n");
> > > +
> > > + /*
> > > + * Use the SBI remote fence extension to avoid
> > > + * the extra context switch needed to handle IPIs.
> > > + */
> > > + static_branch_enable(&riscv_sbi_for_rfence);
> > > }
> > > diff --git a/arch/riscv/kernel/smp.c b/arch/riscv/kernel/smp.c
> > > index 45dd4035416e..8e6eb64459af 100644
> > > --- a/arch/riscv/kernel/smp.c
> > > +++ b/arch/riscv/kernel/smp.c
> > > @@ -171,10 +171,7 @@ bool riscv_ipi_have_virq_range(void)
> > > return (ipi_virq_base) ? true : false;
> > > }
> > >
> > > -DEFINE_STATIC_KEY_FALSE(riscv_ipi_for_rfence);
> > > -EXPORT_SYMBOL_GPL(riscv_ipi_for_rfence);
> > > -
> > > -void riscv_ipi_set_virq_range(int virq, int nr, bool use_for_rfence)
> > > +void riscv_ipi_set_virq_range(int virq, int nr)
> > > {
> > > int i, err;
> > >
> > > @@ -197,12 +194,6 @@ void riscv_ipi_set_virq_range(int virq, int nr,
> > > bool use_for_rfence)
> > >
> > > /* Enabled IPIs for boot CPU immediately */
> > > riscv_ipi_enable();
> > > -
> > > - /* Update RFENCE static key */
> > > - if (use_for_rfence)
> > > - static_branch_enable(&riscv_ipi_for_rfence);
> > > - else
> > > - static_branch_disable(&riscv_ipi_for_rfence);
> > > }
> > >
> > > static const char * const ipi_names[] = {
> > > diff --git a/arch/riscv/mm/cacheflush.c b/arch/riscv/mm/cacheflush.c
> > > index 55a34f2020a8..47c485bc7df0 100644
> > > --- a/arch/riscv/mm/cacheflush.c
> > > +++ b/arch/riscv/mm/cacheflush.c
> > > @@ -21,7 +21,7 @@ void flush_icache_all(void)
> > > {
> > > local_flush_icache_all();
> > >
> > > - if (IS_ENABLED(CONFIG_RISCV_SBI) && !riscv_use_ipi_for_rfence())
> > > + if (riscv_use_sbi_for_rfence())
> > > sbi_remote_fence_i(NULL);
> > > else
> > > on_each_cpu(ipi_remote_fence_i, NULL, 1);
> > > @@ -69,8 +69,7 @@ void flush_icache_mm(struct mm_struct *mm, bool local)
> > > * with flush_icache_deferred().
> > > */
> > > smp_mb();
> > > - } else if (IS_ENABLED(CONFIG_RISCV_SBI) &&
> > > - !riscv_use_ipi_for_rfence()) {
> > > + } else if (riscv_use_sbi_for_rfence()) {
> > > sbi_remote_fence_i(&others);
> > > } else {
> > > on_each_cpu_mask(&others, ipi_remote_fence_i, NULL, 1);
> > > diff --git a/arch/riscv/mm/tlbflush.c b/arch/riscv/mm/tlbflush.c
> > > index 8d12b26f5ac3..0373661bd1c4 100644
> > > --- a/arch/riscv/mm/tlbflush.c
> > > +++ b/arch/riscv/mm/tlbflush.c
> > > @@ -78,10 +78,10 @@ static void __ipi_flush_tlb_all(void *info)
> > >
> > > void flush_tlb_all(void)
> > > {
> > > - if (riscv_use_ipi_for_rfence())
> > > - on_each_cpu(__ipi_flush_tlb_all, NULL, 1);
> > > - else
> > > + if (riscv_use_sbi_for_rfence())
> > > sbi_remote_sfence_vma_asid(NULL, 0, FLUSH_TLB_MAX_SIZE, FLUSH_TLB_NO_ASID);
> > > + else
> > > + on_each_cpu(__ipi_flush_tlb_all, NULL, 1);
> > > }
> > >
> > > struct flush_tlb_range_data {
> > > @@ -102,7 +102,6 @@ static void __flush_tlb_range(struct cpumask
> > > *cmask, unsigned long asid,
> > > unsigned long start, unsigned long size,
> > > unsigned long stride)
> > > {
> > > - struct flush_tlb_range_data ftd;
> > > bool broadcast;
> > >
> > > if (cpumask_empty(cmask))
> > > @@ -118,20 +117,18 @@ static void __flush_tlb_range(struct cpumask
> > > *cmask, unsigned long asid,
> > > broadcast = true;
> > > }
> > >
> > > - if (broadcast) {
> > > - if (riscv_use_ipi_for_rfence()) {
> > > - ftd.asid = asid;
> > > - ftd.start = start;
> > > - ftd.size = size;
> > > - ftd.stride = stride;
> > > - on_each_cpu_mask(cmask,
> > > - __ipi_flush_tlb_range_asid,
> > > - &ftd, 1);
> > > - } else
> > > - sbi_remote_sfence_vma_asid(cmask,
> > > - start, size, asid);
> > > - } else {
> > > + if (!broadcast) {
> > > local_flush_tlb_range_asid(start, size, stride, asid);
> > > + } else if (riscv_use_sbi_for_rfence()) {
> > > + sbi_remote_sfence_vma_asid(cmask, start, size, asid);
> > > + } else {
> > > + struct flush_tlb_range_data ftd;
> > > +
> > > + ftd.asid = asid;
> > > + ftd.start = start;
> > > + ftd.size = size;
> > > + ftd.stride = stride;
> > > + on_each_cpu_mask(cmask, __ipi_flush_tlb_range_asid, &ftd, 1);
> > > }
> > >
> > > if (cmask != cpu_online_mask)
> > > diff --git a/drivers/clocksource/timer-clint.c
> > > b/drivers/clocksource/timer-clint.c
> > > index 09fd292eb83d..0bdd9d7ec545 100644
> > > --- a/drivers/clocksource/timer-clint.c
> > > +++ b/drivers/clocksource/timer-clint.c
> > > @@ -251,7 +251,7 @@ static int __init clint_timer_init_dt(struct
> > > device_node *np)
> > > }
> > >
> > > irq_set_chained_handler(clint_ipi_irq, clint_ipi_interrupt);
> > > - riscv_ipi_set_virq_range(rc, BITS_PER_BYTE, true);
> > > + riscv_ipi_set_virq_range(rc, BITS_PER_BYTE);
> > > clint_clear_ipi();
> > > #endif
> > >
> > > --
> > > 2.43.1
> > >
> > >
> > > _______________________________________________
> > > linux-riscv mailing list
> > > linux-riscv@xxxxxxxxxxxxxxxxxxx
> > > http://lists.infradead.org/mailman/listinfo/linux-riscv
> >

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