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Thu, 18 Nov 2021 16:45:59 +0000 Subject: Re: [PATCH 3/3] drm/amdkfd: simplify drain retry fault To: Felix Kuehling , Philip Yang , amd-gfx@lists.freedesktop.org References: <20211117034324.4663-1-Philip.Yang@amd.com> <20211117034324.4663-3-Philip.Yang@amd.com> <71b8c251-97f3-3064-7861-d2bdb24b6ae9@amd.com> <423b2831-ba9c-3239-574b-ae3ac8ebb112@amd.com> From: philip yang Message-ID: <2425ef80-5129-262d-de53-97d7908cbd94@amd.com> Date: Thu, 18 Nov 2021 11:45:57 -0500 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.13.0 In-Reply-To: <423b2831-ba9c-3239-574b-ae3ac8ebb112@amd.com> Content-Type: text/html; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit X-ClientProxiedBy: BL1PR13CA0199.namprd13.prod.outlook.com (2603:10b6:208:2be::24) To DM4PR12MB5149.namprd12.prod.outlook.com (2603:10b6:5:390::14) MIME-Version: 1.0 Received: from [172.27.226.38] (165.204.55.251) by BL1PR13CA0199.namprd13.prod.outlook.com (2603:10b6:208:2be::24) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.4734.11 via Frontend Transport; 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On 2021-11-18 11:39 a.m., Felix Kuehling wrote:
Am 2021-11-18 um 11:19 a.m. schrieb philip yang:


On 2021-11-17 7:14 p.m., Felix Kuehling wrote:

On 2021-11-16 10:43 p.m., Philip Yang wrote:

unmap range always set svms->drain_pagefaults flag to simplify both
parent range and child range unmap. Deferred list work takes mmap write
lock to read and clear svms->drain_pagefaults, to serialize with unmap
callback.

Add atomic flag svms->draining_faults, if svms->draining_faults is set,
page fault handle ignore the retry fault, to speed up interrupt
handling.

Having both svms->drain_pagefaults and svms->draining_faults is
confusing. Do we really need both? 

Using one flag, I can not find a way to handle the case, unmap new
range. if the flag is set, restore_pages uses the flag to drop fault,
then drain_retry_fault reset the flag after draining is done, we will
not able to drain retry fault for the new range.


I think the correct solution would be to use atomic_inc to set the flag
and atomic_cmp_xchg in svm_range_drain_retry_fault to clear it. If the
flag was changed while svm_range_drain_retry_fault executed, it means
another drain was requested by someone else and the flag should remain
set for another round of draining.

Tanks for the idea to use atomic_cmp_xchg, it will solve the issue.

Philip

Regards,
  Felix



Regards,

Philip


Regards,
  Felix


Signed-off-by: Philip Yang <Philip.Yang@amd.com>
---
  drivers/gpu/drm/amd/amdkfd/kfd_priv.h |  1 +
  drivers/gpu/drm/amd/amdkfd/kfd_svm.c  | 24 ++++++++++++++++++------
  2 files changed, 19 insertions(+), 6 deletions(-)

diff --git a/drivers/gpu/drm/amd/amdkfd/kfd_priv.h
b/drivers/gpu/drm/amd/amdkfd/kfd_priv.h
index 1d3f012bcd2a..4e4640b731e1 100644
--- a/drivers/gpu/drm/amd/amdkfd/kfd_priv.h
+++ b/drivers/gpu/drm/amd/amdkfd/kfd_priv.h
@@ -767,6 +767,7 @@ struct svm_range_list {
      spinlock_t            deferred_list_lock;
      atomic_t            evicted_ranges;
      bool                drain_pagefaults;
+    atomic_t            draining_faults;
      struct delayed_work        restore_work;
      DECLARE_BITMAP(bitmap_supported, MAX_GPU_INSTANCE);
      struct task_struct         *faulting_task;
diff --git a/drivers/gpu/drm/amd/amdkfd/kfd_svm.c
b/drivers/gpu/drm/amd/amdkfd/kfd_svm.c
index 3eb0a9491755..d332462bf9d3 100644
--- a/drivers/gpu/drm/amd/amdkfd/kfd_svm.c
+++ b/drivers/gpu/drm/amd/amdkfd/kfd_svm.c
@@ -1962,6 +1962,7 @@ void svm_range_drain_retry_fault(struct
svm_range_list *svms)
        p = container_of(svms, struct kfd_process, svms);
  +    atomic_set(&svms->draining_faults, 1);
      for_each_set_bit(i, svms->bitmap_supported, p->n_pdds) {
          pdd = p->pdds[i];
          if (!pdd)
@@ -1975,6 +1976,7 @@ void svm_range_drain_retry_fault(struct
svm_range_list *svms)
          flush_work(&adev->irq.ih1_work);
          pr_debug("drain retry fault gpu %d svms 0x%p done\n", i,
svms);
      }
+    atomic_set(&svms->draining_faults, 0);
  }
    static void svm_range_deferred_list_work(struct work_struct *work)
@@ -2002,6 +2004,7 @@ static void
svm_range_deferred_list_work(struct work_struct *work)
       * mmap write lock to serialize with munmap notifiers.
       */
      if (unlikely(READ_ONCE(svms->drain_pagefaults))) {
+        atomic_set(&svms->draining_faults, 1);
          WRITE_ONCE(svms->drain_pagefaults, false);
          mmap_write_unlock(mm);
          svm_range_drain_retry_fault(svms);
@@ -2049,12 +2052,6 @@ svm_range_add_list_work(struct svm_range_list
*svms, struct svm_range *prange,
              struct mm_struct *mm, enum svm_work_list_ops op)
  {
      spin_lock(&svms->deferred_list_lock);
-    /* Make sure pending page faults are drained in the deferred
worker
-     * before the range is freed to avoid straggler interrupts on
-     * unmapped memory causing "phantom faults".
-     */
-    if (op == SVM_OP_UNMAP_RANGE)
-        svms->drain_pagefaults = true;
      /* if prange is on the deferred list */
      if (!list_empty(&prange->deferred_list)) {
          pr_debug("update exist prange 0x%p work op %d\n", prange,
op);
@@ -2133,6 +2130,13 @@ svm_range_unmap_from_cpu(struct mm_struct
*mm, struct svm_range *prange,
      pr_debug("svms 0x%p prange 0x%p [0x%lx 0x%lx] [0x%lx
0x%lx]\n", svms,
           prange, prange->start, prange->last, start, last);
  +    /* Make sure pending page faults are drained in the deferred
worker
+     * before the range is freed to avoid straggler interrupts on
+     * unmapped memory causing "phantom faults".
+     */
+    pr_debug("set range drain_pagefaults true\n");
+    WRITE_ONCE(svms->drain_pagefaults, true);
+
      unmap_parent = start <= prange->start && last >= prange->last;
        list_for_each_entry(pchild, &prange->child_list, child_list) {
@@ -2595,6 +2599,13 @@ svm_range_restore_pages(struct amdgpu_device
*adev, unsigned int pasid,
      mm = p->mm;
      mmap_write_lock(mm);
  +    if (!!atomic_read(&svms->draining_faults) ||
+        READ_ONCE(svms->drain_pagefaults)) {
+        pr_debug("draining retry fault, drop fault 0x%llx\n", addr);
+        mmap_write_downgrade(mm);
+        goto out_unlock_mm;
+    }
+
      vma = find_vma(p->mm, addr << PAGE_SHIFT);
      if (!vma || (addr << PAGE_SHIFT) < vma->vm_start) {
          pr_debug("VMA not found for address 0x%llx\n", addr);
@@ -2732,6 +2743,7 @@ int svm_range_list_init(struct kfd_process *p)
      mutex_init(&svms->lock);
      INIT_LIST_HEAD(&svms->list);
      atomic_set(&svms->evicted_ranges, 0);
+    atomic_set(&svms->draining_faults, 0);
      INIT_DELAYED_WORK(&svms->restore_work, svm_range_restore_work);
      INIT_WORK(&svms->deferred_list_work,
svm_range_deferred_list_work);
      INIT_LIST_HEAD(&svms->deferred_range_list);