Re: [PATCH v2] RISC-V: Implement ASID allocator

From: Gary Guo
Date: Thu Mar 28 2019 - 10:30:42 EST



On 28/03/2019 14:09, Anup Patel wrote:
> On Thu, Mar 28, 2019 at 7:07 PM Gary Guo <gary@xxxxxxxxxxx> wrote:
>>
>> Hi Anup,
>>
>> The code still does not use ASID in TLB flush routines. Without this
>> added the code does not boot on systems with true ASID support.
>
> Can you elaborate why flush by ASID is need and flush_tlb_all() will
> not work?
>
flush_tlb_all() will work, but not flush_tlb_mm, flush_tlb_page,
flush_tlb_range. When we want to flush something related to a MM we need
to get its ASID and SFENCE with that ASID.
>>
>> We also need to consider the case of CONTEXTID overflow on 32-bit
>> systems. 32-bit CONTEXTID may overflow in a month time.
>
> On 32bit systems, upper 24bits of CONTEXTID will be VERSION and
> lower 8bits will be HW ASID.
>
> Can you elaborate how did you reach to conclusion that CONTEXID
> will overflow in a month time?
>
Assume a case where we have 256 processes to run, and 8 cores,
2^32/(250Hz)/8 = 24 days.
>>
>> Please all see my inline comments.
>>
>> Best,
>> Gary
>>
>> On 28/03/2019 06:32, 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 every 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. This means last ASID is always reserved
>>> because it is used for initial MM context.
>>>
>>> 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
>>>
>>> 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 and SiFive Unleashed board.
>>> On QEMU/virt machine, we see 10% (approx) performance improvement with
>>> SW emulated TLBs provided by QEMU. Unfortunately, ASID bits of SATP CSR
>>> are not implemented on SiFive Unleashed board so we don't see any change
>>> in performance.
>>>
>>> Signed-off-by: Gary Guo <gary@xxxxxxxxxxx>
>> Could you add a Co-developed-by line in addition to Signed-off-by as
>> well? Thanks.
>
> Sure, I will add.
>
>>> Signed-off-by: Anup Patel <anup.patel@xxxxxxx>
>>> ---
>>> 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-5.1-rc2 and TLB flush cleanup patches v4
>>> from Gary Guo. It can be also found in riscv_asid_allocator_v2 branch
>>> of https://github.com/avpatel/linux.git
>>> ---
>>> arch/riscv/include/asm/csr.h | 6 +
>>> arch/riscv/include/asm/mmu.h | 1 +
>>> arch/riscv/include/asm/mmu_context.h | 1 +
>>> arch/riscv/kernel/head.S | 2 +
>>> arch/riscv/mm/context.c | 249 +++++++++++++++++++++++++--
>>> 5 files changed, 247 insertions(+), 12 deletions(-)
>>>
>>> diff --git a/arch/riscv/include/asm/csr.h b/arch/riscv/include/asm/csr.h
>>> index 28a0d1cb374c..ce18ab8f53ed 100644
>>> --- a/arch/riscv/include/asm/csr.h
>>> +++ b/arch/riscv/include/asm/csr.h
>>> @@ -45,10 +45,16 @@
>>> #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
>>>
>>> /* Interrupt Enable and Interrupt Pending flags */
>>> diff --git a/arch/riscv/include/asm/mmu.h b/arch/riscv/include/asm/mmu.h
>>> index 5df2dccdba12..42a9ca0fe1fb 100644
>>> --- a/arch/riscv/include/asm/mmu.h
>>> +++ b/arch/riscv/include/asm/mmu.h
>>> @@ -18,6 +18,7 @@
>>> #ifndef __ASSEMBLY__
>>>
>>> typedef struct {
>>> + atomic_long_t id;
>>> void *vdso;
>>> #ifdef CONFIG_SMP
>>> /* A local icache flush is needed before user execution can resume. */
>>> diff --git a/arch/riscv/include/asm/mmu_context.h b/arch/riscv/include/asm/mmu_context.h
>>> index bf4f097a9051..ba6ab35c18dc 100644
>>> --- a/arch/riscv/include/asm/mmu_context.h
>>> +++ b/arch/riscv/include/asm/mmu_context.h
>>> @@ -30,6 +30,7 @@ static inline void enter_lazy_tlb(struct mm_struct *mm,
>>> static inline int init_new_context(struct task_struct *task,
>>> struct mm_struct *mm)
>>> {
>>> + atomic_long_set(&(mm)->context.id, 0);
>> Parenthesis around mm isn't necessary
>
> Okay, I will update.
>
>>> return 0;
>>> }
>>>
>>> diff --git a/arch/riscv/kernel/head.S b/arch/riscv/kernel/head.S
>>> index fe884cd69abd..c3f9adc0d054 100644
>>> --- a/arch/riscv/kernel/head.S
>>> +++ b/arch/riscv/kernel/head.S
>>> @@ -95,6 +95,8 @@ relocate:
>>> la a2, swapper_pg_dir
>>> srl a2, a2, PAGE_SHIFT
>>> li a1, SATP_MODE
>>> + li a0, (SATP_ASID_MASK << SATP_ASID_SHIFT)
>>> + or a1, a1, a0
>>> or a2, a2, a1
>>>
>>> /*
>>> diff --git a/arch/riscv/mm/context.c b/arch/riscv/mm/context.c
>>> index 0f787bcd3a7a..1205d33d1b1b 100644
>>> --- a/arch/riscv/mm/context.c
>>> +++ b/arch/riscv/mm/context.c
>>> @@ -2,13 +2,209 @@
>>> /*
>>> * Copyright (C) 2012 Regents of the University of California
>>> * Copyright (C) 2017 SiFive
>>> + * Copyright (C) 2019 Western Digital Corporation or its affiliates.
>>> */
>>>
>>> +#include <linux/bitops.h>
>>> #include <linux/mm.h>
>>> +#include <linux/slab.h>
>>>
>>> #include <asm/tlbflush.h>
>>> #include <asm/cacheflush.h>
>>>
>>> +static bool use_asid_allocator;
>>> +static unsigned long asid_bits;
>>> +static unsigned long num_asids;
>>> +static unsigned long asid_mask;
>>> +static unsigned long first_version;
>>> +
>>> +static atomic_long_t current_version;
>>> +
>>> +static DEFINE_RAW_SPINLOCK(context_lock);
>>> +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 */
>>> +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 acitve 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 last ASID as used because it is used at boot-time */
>>> + __set_bit(asid_mask, context_asid_map);
>> Looks unnecessary as we always start find_next_zero_bit from idx 1.
>
> This is to ensure that we never use last ASID >
Uh, sorry. I misread. But we surely can use the last ASID after the
first rollover?
>>> +} >>> +
>>> +/* Note: must be called with context_lock held */
>>> +static unsigned long __new_context(struct mm_struct *mm,
>>> + bool *need_tlb_flush)
>>> +{
>>> + 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);
>> Shouldn't this be cntx & asid_mask ?
>
> Ahh, typo. Thanks for catching.
>
>>> +
>>> + /*
>>> + * 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(first_version, &current_version);
>>> +
>>> + /* Flush everything */
>>> + __flush_context();
>>> + *need_tlb_flush = true;
>>> +
>>> + /* 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_tlb_flush = 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 ^ atomic_long_read(&current_version)) >> asid_bits) &&
>> This looks hard to read. Probably
>> (cntx &~ asid_mask) == atomic_long_read(&current_version)
>> is clearer.
>
> No issues, I am fine with either way. I will update.
>
>>> + atomic_long_cmpxchg_relaxed(&per_cpu(active_context, cpu),
>>> + old_active_cntx, cntx))
>>> + goto switch_mm_fast;
>>> +
>>> + raw_spin_lock_irqsave(&context_lock, flags);
>> Any reason that we prefer raw_ here?
>>> +
>>> + /* Check that our ASID belongs to the current_version. */
>>> + cntx = atomic_long_read(&mm->context.id);
>>> + if ((cntx ^ atomic_long_read(&current_version)) >> asid_bits) {
>> Same as above, probably
>> (cntx &~ asid_mask) != atomic_long_read(&current_version)
>> makes more sense.
>>> + cntx = __new_context(mm, &need_tlb_flush);
>>> + atomic_long_set(&mm->context.id, cntx);
>>> + }
>>> +
>>> + atomic_long_set(&per_cpu(active_context, cpu), cntx);
>>> +
>>> + raw_spin_unlock_irqrestore(&context_lock, flags);
>>> +
>>> +switch_mm_fast:
>>> + /*
>>> + * Use the old spbtr name instead of using the current satp
>>> + * name to support binutils 2.29 which doesn't know about the
>>> + * privileged ISA 1.10 yet.
>>> + */
>>> + csr_write(sptbr, virt_to_pfn(mm->pgd) |
>>> + ((cntx & asid_mask) << SATP_ASID_SHIFT) |
>>> + SATP_MODE);
>>> +
>>> + if (need_tlb_flush)
>>> + flush_tlb_all();
>> I think your intention here is to avoid calling flush_tlb_all when IRQs
>> are off in the critical region. However, switch_mm will be called from
>> scheduler as well which also turn irqs off, so this still cause issue. I
>> think a better way is to force flush_tlb_all to use SBI when IRQs are
>> off. What do you think?
>
> We are still waiting for OpenSBI to provide complete implementation.
>
> I agree that we should prefer SBI based remote TLB flush all here. Let's
> wait for more comments.
>
I prefer SBI as well. Can you reopen OpenSBI issue #87 to track the
progress until we can proper handle race conditions in OpenSBI? Once
that's completed I'll drop the IPI patch and we can safely do
flush_tlb_all within __flush_context.
>>> +}
>>> +
>>> +static void set_mm_noasid(struct mm_struct *mm)
>>> +{
>>> + /*
>>> + * Use the old spbtr name instead of using the current satp
>>> + * name to support binutils 2.29 which doesn't know about the
>>> + * privileged ISA 1.10 yet.
>>> + */
>>> + csr_write(sptbr, virt_to_pfn(mm->pgd) | SATP_MODE);
>>> +
>>> + /*
>>> + * sfence.vma after SATP write. We call it on MM context instead of
>>> + * calling local_flush_tlb_all to prevent global mappings from being
>>> + * affected.
>>> + */
>>> + local_flush_tlb_mm(mm);
>>> +}
>>> +
>>> /*
>>> * 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
>>> @@ -58,20 +254,49 @@ 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));
>>>
>>> - /*
>>> - * Use the old spbtr name instead of using the current satp
>>> - * name to support binutils 2.29 which doesn't know about the
>>> - * privileged ISA 1.10 yet.
>>> - */
>>> - csr_write(sptbr, virt_to_pfn(next->pgd) | SATP_MODE);
>>> + if (use_asid_allocator)
>>> + set_mm_asid(next, cpu);
>>> + else
>>> + set_mm_noasid(next);
>>> +
>>> + flush_icache_deferred(next);
>>> +}
>>> +
>>> +static int asids_init(void)
>>> +{
>>> + /* Figure-out number of ASID bits in HW */
>>> + asid_bits = (csr_read(sptbr) >> SATP_ASID_SHIFT) & SATP_ASID_MASK;
>>> + asid_bits = fls_long(asid_bits);
>>> +
>>> + /* Pre-compute ASID details */
>>> + num_asids = 1 << asid_bits;
>>> + asid_mask = num_asids - 1;
>>> + first_version = num_asids;
>> Is there any reason we want to have two set-once variables with same value?
>
> Yap, "first_version" looks redundant. I will update.
>
>>>
>>> /*
>>> - * sfence.vma after SATP write. We call it on MM context instead of
>>> - * calling local_flush_tlb_all to prevent global mappings from being
>>> - * affected.
>>> + * Use ASID allocator only if number of HW ASIDs are
>>> + * at-least twice more than CPUs
>>> */
>>> - local_flush_tlb_mm(next);
>>> + use_asid_allocator =
>>> + (num_asids <= (2 * num_possible_cpus())) ? false : true;
>>>
>>> - flush_icache_deferred(next);
>>> -}
>>> + /* Setup ASID allocator if available */
>>> + if (use_asid_allocator) {
>>> + atomic_long_set(&current_version, first_version);
>>>
>>> + 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(asid_mask, context_asid_map);
>>> +
>>> + pr_info("ASID allocator using %lu entries\n", num_asids);
>>> + } else {
>> If we decide not to use ASID allocator, we will need to set ASID field
>> to zero on *all harts* before we do our first switch_mm. Otherwise we
>> will end up a hart running non-zero ASID and another running zero ASID
>> with different page table.
>
> Yes, I saw that in your implementation but for better readability and
> debugability. I have preserved asid_bits that we computed and added
> separate use_asid_allocator flag.
I didn't say I'm against having use_asid_allocator.
>
> In future, I plan to show asid_bits in /proc/cpuinfo as-well.
>
>>> + pr_info("ASID allocator disabled\n");
>>> + }
>>> +
>>> + return 0;
>>> +}
>>> +early_initcall(asids_init);
>>>
>
> Regards,
> Anup
>