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From:   Anup Patel <anup@brainfault.org>
Date:   Wed, 3 Feb 2021 10:55:42 +0530
Message-ID: <CAAhSdy14_WM7ejGPHVCg3Y0Us5b7jcDq4nPw_rmtfXy6ZnTjwQ@mail.gmail.com>
Subject: Re: [PATCH v4] RISC-V: Implement ASID allocator
To:     Palmer Dabbelt <palmer@dabbelt.com>
Cc:     Anup Patel <Anup.Patel@wdc.com>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        Albert Ou <aou@eecs.berkeley.edu>,
        Atish Patra <Atish.Patra@wdc.com>,
        Alistair Francis <Alistair.Francis@wdc.com>,
        linux-riscv <linux-riscv@lists.infradead.org>,
        "linux-kernel@vger.kernel.org List" <linux-kernel@vger.kernel.org>
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On Wed, Feb 3, 2021 at 7:58 AM Palmer Dabbelt <palmer@dabbelt.com> wrote:
>
> On Thu, 21 Jan 2021 05:50:16 PST (-0800), Anup Patel wrote:
> > Currently, we do local TLB flush on every MM switch. This is very harsh on
> > performance because we are forcing page table walks after every MM switch.
> >
> > This patch implements ASID allocator for assigning an ASID to a MM context.
> > The number of ASIDs are limited in HW so we create a logical entity named
> > CONTEXTID for assigning to MM context. The lower bits of CONTEXTID are ASID
> > and upper bits are VERSION number. The number of usable ASID bits supported
> > by HW are detected at boot-time by writing 1s to ASID bits in SATP CSR.
> >
> > We allocate new CONTEXTID on first MM switch for a MM context where the
> > ASID is allocated from an ASID bitmap and VERSION is provide by an atomic
> > counter. At time of allocating new CONTEXTID, if we run out of available
> > ASIDs then:
> > 1. We flush the ASID bitmap
> > 2. Increment current VERSION atomic counter
> > 3. Re-allocate ASID from ASID bitmap
> > 4. Flush TLB on all CPUs
> > 5. Try CONTEXTID re-assignment on all CPUs
> >
> > Please note that we don't use ASID #0 because it is used at boot-time by
> > all CPUs for initial MM context. Also, newly created context is always
> > assigned CONTEXTID #0 (i.e. VERSION #0 and ASID #0) which is an invalid
> > context in our implementation.
> >
> > Using above approach, we have virtually infinite CONTEXTIDs on-top-of
> > limited number of HW ASIDs. This approach is inspired from ASID allocator
> > used for Linux ARM/ARM64 but we have adapted it for RISC-V. Overall, this
> > ASID allocator helps us reduce rate of local TLB flushes on every CPU
> > thereby increasing performance.
> >
> > This patch is tested on QEMU virt machine, Spike and SiFive Unleashed
> > board. On QEMU virt machine, we see some (3-5% approx) performance
> > improvement with SW emulated TLBs provided by QEMU. Unfortunately,
> > the ASID bits of the SATP CSR are not implemented on Spike and SiFive
> > Unleashed board so we don't see any change in performance. On real
> > HW having all ASID bits implemented, the performance gains will be
> > much more due improved sharing of TLB among different processes.
> >
> > Signed-off-by: Anup Patel <anup.patel@wdc.com>
> > ---
> > Changes since v3:
> > - Rebased on Linux-5.11-rc4. The previous v3 patch (almost 2 years back)
> >   was basd on Linux-5.1-rc2
> > - Updated implementation to consider NoMMU kernel
> > - Converted use_asid_allocator boolean flag into static key
> > - Improved boot-time print in asids_init() to show number of ASID bits
> > - Access SATP CSR by number instead of old CSR name "sptbr"
> >
> > Changes since v2:
> > - Move to lazy TLB flushing because we get slow path warnings if we
> >   use flush_tlb_all()
> > - Don't set ASID bits to all 1s in head.s. Instead just do it on
> >   boot CPU calling asids_init() for determining number of HW ASID bits
> > - Make CONTEXT version comparison more readable in set_mm_asid()
> > - Fix typo in __flush_context()
> >
> > Changes since v1:
> > - We adapt good aspects from Gary Guo's ASID allocator implementation
> >   and provide due credit to him by adding his SoB.
> > - Track ASIDs active during context flush and mark them as reserved
> > - Set ASID bits to all 1s to simplify number of ASID bit detection
> > - Use atomic_long_t instead of atomic64_t for being 32bit friendly
> > - Use unsigned long instead of u64 for being 32bit friendly
> > - Use flush_tlb_all() instead of lazy local_tlb_flush_all() at time
> >   of context flush
> >
> > This patch is based on Linux-5.11-rc4 and can be found in the
> > riscv_asid_allocator_v4 branch of https://github.com/avpatel/linux.git
> > ---
> >  arch/riscv/include/asm/csr.h         |   6 +
> >  arch/riscv/include/asm/mmu.h         |   2 +
> >  arch/riscv/include/asm/mmu_context.h |  10 +
> >  arch/riscv/mm/context.c              | 261 ++++++++++++++++++++++++++-
> >  4 files changed, 275 insertions(+), 4 deletions(-)
> >
> > diff --git a/arch/riscv/include/asm/csr.h b/arch/riscv/include/asm/csr.h
> > index cec462e198ce..caadfc1d7487 100644
> > --- a/arch/riscv/include/asm/csr.h
> > +++ b/arch/riscv/include/asm/csr.h
> > @@ -41,10 +41,16 @@
> >  #define SATP_PPN     _AC(0x003FFFFF, UL)
> >  #define SATP_MODE_32 _AC(0x80000000, UL)
> >  #define SATP_MODE    SATP_MODE_32
> > +#define SATP_ASID_BITS       9
> > +#define SATP_ASID_SHIFT      22
> > +#define SATP_ASID_MASK       _AC(0x1FF, UL)
> >  #else
> >  #define SATP_PPN     _AC(0x00000FFFFFFFFFFF, UL)
> >  #define SATP_MODE_39 _AC(0x8000000000000000, UL)
> >  #define SATP_MODE    SATP_MODE_39
> > +#define SATP_ASID_BITS       16
> > +#define SATP_ASID_SHIFT      44
> > +#define SATP_ASID_MASK       _AC(0xFFFF, UL)
> >  #endif
> >
> >  /* Exception cause high bit - is an interrupt if set */
> > diff --git a/arch/riscv/include/asm/mmu.h b/arch/riscv/include/asm/mmu.h
> > index dabcf2cfb3dc..0099dc116168 100644
> > --- a/arch/riscv/include/asm/mmu.h
> > +++ b/arch/riscv/include/asm/mmu.h
> > @@ -12,6 +12,8 @@
> >  typedef struct {
> >  #ifndef CONFIG_MMU
> >       unsigned long   end_brk;
> > +#else
> > +     atomic_long_t id;
> >  #endif
> >       void *vdso;
> >  #ifdef CONFIG_SMP
> > diff --git a/arch/riscv/include/asm/mmu_context.h b/arch/riscv/include/asm/mmu_context.h
> > index 250defa06f3a..b0659413a080 100644
> > --- a/arch/riscv/include/asm/mmu_context.h
> > +++ b/arch/riscv/include/asm/mmu_context.h
> > @@ -23,6 +23,16 @@ static inline void activate_mm(struct mm_struct *prev,
> >       switch_mm(prev, next, NULL);
> >  }
> >
> > +#define init_new_context init_new_context
> > +static inline int init_new_context(struct task_struct *tsk,
> > +                     struct mm_struct *mm)
> > +{
> > +#ifdef CONFIG_MMU
> > +     atomic_long_set(&mm->context.id, 0);
> > +#endif
> > +     return 0;
> > +}
> > +
> >  #include <asm-generic/mmu_context.h>
> >
> >  #endif /* _ASM_RISCV_MMU_CONTEXT_H */
> > diff --git a/arch/riscv/mm/context.c b/arch/riscv/mm/context.c
> > index 613ec81a8979..6216fed8f9cc 100644
> > --- a/arch/riscv/mm/context.c
> > +++ b/arch/riscv/mm/context.c
> > @@ -2,13 +2,269 @@
> >  /*
> >   * Copyright (C) 2012 Regents of the University of California
> >   * Copyright (C) 2017 SiFive
> > + * Copyright (C) 2021 Western Digital Corporation or its affiliates.
> >   */
> >
> > +#include <linux/bitops.h>
> > +#include <linux/cpumask.h>
> >  #include <linux/mm.h>
> > +#include <linux/percpu.h>
> > +#include <linux/slab.h>
> > +#include <linux/spinlock.h>
> > +#include <linux/static_key.h>
> >  #include <asm/tlbflush.h>
> >  #include <asm/cacheflush.h>
> >  #include <asm/mmu_context.h>
> >
> > +#ifdef CONFIG_MMU
> > +
> > +static DEFINE_STATIC_KEY_FALSE(use_asid_allocator);
> > +
> > +static unsigned long asid_bits;
> > +static unsigned long num_asids;
> > +static unsigned long asid_mask;
> > +
> > +static atomic_long_t current_version;
> > +
> > +static DEFINE_RAW_SPINLOCK(context_lock);
> > +static cpumask_t context_tlb_flush_pending;
> > +static unsigned long *context_asid_map;
> > +
> > +static DEFINE_PER_CPU(atomic_long_t, active_context);
> > +static DEFINE_PER_CPU(unsigned long, reserved_context);
> > +
> > +static bool check_update_reserved_context(unsigned long cntx,
> > +                                       unsigned long newcntx)
> > +{
> > +     int cpu;
> > +     bool hit = false;
> > +
> > +     /*
> > +      * Iterate over the set of reserved CONTEXT looking for a match.
> > +      * If we find one, then we can update our mm to use new CONTEXT
> > +      * (i.e. the same CONTEXT in the current_version) but we can't
> > +      * exit the loop early, since we need to ensure that all copies
> > +      * of the old CONTEXT are updated to reflect the mm. Failure to do
> > +      * so could result in us missing the reserved CONTEXT in a future
> > +      * version.
> > +      */
> > +     for_each_possible_cpu(cpu) {
> > +             if (per_cpu(reserved_context, cpu) == cntx) {
> > +                     hit = true;
> > +                     per_cpu(reserved_context, cpu) = newcntx;
> > +             }
> > +     }
> > +
> > +     return hit;
> > +}
> > +
> > +/* Note: must be called with context_lock held */
>
> These are usually better expressed as lockdep assertions.  I don't see any
> reason why they wouldn't work here.

Okay, I will add a lockdep assertion here.

>
> > +static void __flush_context(void)
> > +{
> > +     int i;
> > +     unsigned long cntx;
> > +
> > +     /* Update the list of reserved ASIDs and the ASID bitmap. */
> > +     bitmap_clear(context_asid_map, 0, num_asids);
> > +
> > +     /* Mark already active ASIDs as used */
> > +     for_each_possible_cpu(i) {
> > +             cntx = atomic_long_xchg_relaxed(&per_cpu(active_context, i), 0);
> > +             /*
> > +              * If this CPU has already been through a rollover, but
> > +              * hasn't run another task in the meantime, we must preserve
> > +              * its reserved CONTEXT, as this is the only trace we have of
> > +              * the process it is still running.
> > +              */
> > +             if (cntx == 0)
> > +                     cntx = per_cpu(reserved_context, i);
> > +
> > +             __set_bit(cntx & asid_mask, context_asid_map);
> > +             per_cpu(reserved_context, i) = cntx;
> > +     }
> > +
> > +     /* Mark ASID #0 as used because it is used at boot-time */
> > +     __set_bit(0, context_asid_map);
> > +
> > +     /* Queue a TLB invalidation for each CPU on next context-switch */
> > +     cpumask_setall(&context_tlb_flush_pending);
> > +}
> > +
> > +/* Note: must be called with context_lock held */
> > +static unsigned long __new_context(struct mm_struct *mm)
> > +{
> > +     static u32 cur_idx = 1;
> > +     unsigned long cntx = atomic_long_read(&mm->context.id);
> > +     unsigned long asid, ver = atomic_long_read(&current_version);
> > +
> > +     if (cntx != 0) {
> > +             unsigned long newcntx = ver | (cntx & asid_mask);
> > +
> > +             /*
> > +              * If our current CONTEXT was active during a rollover, we
> > +              * can continue to use it and this was just a false alarm.
> > +              */
> > +             if (check_update_reserved_context(cntx, newcntx))
> > +                     return newcntx;
> > +
> > +             /*
> > +              * We had a valid CONTEXT in a previous life, so try to
> > +              * re-use it if possible.
> > +              */
> > +             if (!__test_and_set_bit(cntx & asid_mask, context_asid_map))
> > +                     return newcntx;
> > +     }
> > +
> > +     /*
> > +      * Allocate a free ASID. If we can't find one then increment
> > +      * current_version and flush all ASIDs.
> > +      */
> > +     asid = find_next_zero_bit(context_asid_map, num_asids, cur_idx);
> > +     if (asid != num_asids)
> > +             goto set_asid;
> > +
> > +     /* We're out of ASIDs, so increment current_version */
> > +     ver = atomic_long_add_return_relaxed(num_asids, &current_version);
> > +
> > +     /* Flush everything  */
> > +     __flush_context();
> > +
> > +     /* We have more ASIDs than CPUs, so this will always succeed */
> > +     asid = find_next_zero_bit(context_asid_map, num_asids, 1);
> > +
> > +set_asid:
> > +     __set_bit(asid, context_asid_map);
> > +     cur_idx = asid;
> > +     return asid | ver;
> > +}
> > +
> > +static void set_mm_asid(struct mm_struct *mm, unsigned int cpu)
> > +{
> > +     unsigned long flags;
> > +     bool need_flush_tlb = false;
> > +     unsigned long cntx, old_active_cntx;
> > +
> > +     cntx = atomic_long_read(&mm->context.id);
> > +
> > +     /*
> > +      * If our active_context is non-zero and the context matches the
> > +      * current_version, then we update the active_context entry with a
> > +      * relaxed cmpxchg.
> > +      *
> > +      * Following is how we handle racing with a concurrent rollover:
> > +      *
> > +      * - We get a zero back from the cmpxchg and end up waiting on the
> > +      *   lock. Taking the lock synchronises with the rollover and so
> > +      *   we are forced to see the updated verion.
> > +      *
> > +      * - We get a valid context back from the cmpxchg then we continue
> > +      *   using old ASID because __flush_context() would have marked ASID
> > +      *   of active_context as used and next context switch we will
> > +      *   allocate new context.
> > +      */
> > +     old_active_cntx = atomic_long_read(&per_cpu(active_context, cpu));
> > +     if (old_active_cntx &&
> > +         ((cntx & ~asid_mask) == atomic_long_read(&current_version)) &&
> > +         atomic_long_cmpxchg_relaxed(&per_cpu(active_context, cpu),
> > +                                     old_active_cntx, cntx))
> > +             goto switch_mm_fast;
> > +
> > +     raw_spin_lock_irqsave(&context_lock, flags);
> > +
> > +     /* Check that our ASID belongs to the current_version. */
> > +     cntx = atomic_long_read(&mm->context.id);
> > +     if ((cntx & ~asid_mask) != atomic_long_read(&current_version)) {
> > +             cntx = __new_context(mm);
> > +             atomic_long_set(&mm->context.id, cntx);
> > +     }
> > +
> > +     if (cpumask_test_and_clear_cpu(cpu, &context_tlb_flush_pending))
> > +             need_flush_tlb = true;
> > +
> > +     atomic_long_set(&per_cpu(active_context, cpu), cntx);
> > +
> > +     raw_spin_unlock_irqrestore(&context_lock, flags);
> > +
> > +switch_mm_fast:
> > +     csr_write(CSR_SATP, virt_to_pfn(mm->pgd) |
> > +               ((cntx & asid_mask) << SATP_ASID_SHIFT) |
> > +               SATP_MODE);
> > +
> > +     if (need_flush_tlb)
> > +             local_flush_tlb_all();
> > +}
> > +
> > +static void set_mm_noasid(struct mm_struct *mm)
> > +{
> > +     /* Switch the page table and blindly nuke entire local TLB */
> > +     csr_write(CSR_SATP, virt_to_pfn(mm->pgd) | SATP_MODE);
> > +     local_flush_tlb_all();
> > +}
> > +
> > +static inline void set_mm(struct mm_struct *mm, unsigned int cpu)
> > +{
> > +     if (static_branch_unlikely(&use_asid_allocator))
> > +             set_mm_asid(mm, cpu);
> > +     else
> > +             set_mm_noasid(mm);
> > +}
> > +
> > +static int asids_init(void)
> > +{
> > +     unsigned long old;
> > +
> > +     /* Figure-out number of ASID bits in HW */
> > +     old = csr_read(CSR_SATP);
> > +     asid_bits = old | (SATP_ASID_MASK << SATP_ASID_SHIFT);
> > +     csr_write(CSR_SATP, asid_bits);
> > +     asid_bits = (csr_read(CSR_SATP) >> SATP_ASID_SHIFT)  & SATP_ASID_MASK;
> > +     asid_bits = fls_long(asid_bits);
> > +     csr_write(CSR_SATP, old);
> > +
> > +     /*
> > +      * In the process of determining number of ASID bits (above)
> > +      * we polluted the TLB of current HART so let's do TLB flushed
> > +      * to remove unwanted TLB enteries.
> > +      */
> > +     local_flush_tlb_all();
> > +
> > +     /* Pre-compute ASID details */
> > +     num_asids = 1 << asid_bits;
> > +     asid_mask = num_asids - 1;
> > +
> > +     /*
> > +      * Use ASID allocator only if number of HW ASIDs are
> > +      * at-least twice more than CPUs
> > +      */
> > +     if (num_asids > (2 * num_possible_cpus())) {
> > +             atomic_long_set(&current_version, num_asids);
> > +
> > +             context_asid_map = kcalloc(BITS_TO_LONGS(num_asids),
> > +                                sizeof(*context_asid_map), GFP_KERNEL);
> > +             if (!context_asid_map)
> > +                     panic("Failed to allocate bitmap for %lu ASIDs\n",
> > +                           num_asids);
> > +
> > +             __set_bit(0, context_asid_map);
> > +
> > +             static_branch_enable(&use_asid_allocator);
> > +
> > +             pr_info("ASID allocator using %lu bits (%lu entries)\n",
> > +                     asid_bits, num_asids);
> > +     } else {
> > +             pr_info("ASID allocator disabled\n");
> > +     }
> > +
> > +     return 0;
> > +}
> > +early_initcall(asids_init);
> > +#else
> > +static inline void set_mm(struct mm_struct *mm, unsigned int cpu)
> > +{
> > +     /* Nothing to do here when there is no MMU */
> > +}
> > +#endif
> > +
> >  /*
> >   * When necessary, performs a deferred icache flush for the given MM context,
> >   * on the local CPU.  RISC-V has no direct mechanism for instruction cache
> > @@ -58,10 +314,7 @@ void switch_mm(struct mm_struct *prev, struct mm_struct *next,
> >       cpumask_clear_cpu(cpu, mm_cpumask(prev));
> >       cpumask_set_cpu(cpu, mm_cpumask(next));
> >
> > -#ifdef CONFIG_MMU
> > -     csr_write(CSR_SATP, virt_to_pfn(next->pgd) | SATP_MODE);
> > -     local_flush_tlb_all();
> > -#endif
> > +     set_mm(next, cpu);
> >
> >       flush_icache_deferred(next);
> >  }
>
> So I know we'd said before that we weren't going to take this until there's
> hardware, but I think the QEMU support is good enough -- I don't really care if
> the ISA says this might change, it's been in there for long enough.
>
> Aside from the assertions
>
> Reviewed-by: Palmer Dabbelt <palmerdabbelt@google.com>
>
> LMK if you're going to send a v5 or you want me to just fix it up.

I will send v5 in the next few hours. Thanks for the review.

Best Regards,
Anup