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Re: [PATCH] ratelimit: protect with a mutex


From: Emanuele Giuseppe Esposito
Subject: Re: [PATCH] ratelimit: protect with a mutex
Date: Wed, 14 Apr 2021 13:22:27 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.8.1



On 13/04/2021 14:55, Paolo Bonzini wrote:
Right now, rate limiting is protected by the AioContext mutex, which is
taken for example both by the block jobs and by qmp_block_job_set_speed
(via find_block_job).

We would like to remove the dependency of block layer code on the
AioContext mutex, since most drivers and the core I/O code are already
not relying on it.  However, there is no existing lock that can easily
be taken by both ratelimit_set_speed and ratelimit_calculate_delay,
especially because the latter might run in coroutine context (and
therefore under a CoMutex) but the former will not.

Since concurrent calls to ratelimit_calculate_delay are not possible,
one idea could be to use a seqlock to get a snapshot of slice_ns and
slice_quota.  But for now keep it simple, and just add a mutex to the
RateLimit struct; block jobs are generally not performance critical to
the point of optimizing the clock cycles spent in synchronization.

This also requires the introduction of init/destroy functions, so
add them to the two users of ratelimit.h.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
  block/block-copy.c       |  2 ++
  blockjob.c               |  3 +++
  include/qemu/ratelimit.h | 14 ++++++++++++++
  3 files changed, 19 insertions(+)

diff --git a/block/block-copy.c b/block/block-copy.c
index 39ae481c8b..9b4af00614 100644
--- a/block/block-copy.c
+++ b/block/block-copy.c
@@ -230,6 +230,7 @@ void block_copy_state_free(BlockCopyState *s)
          return;
      }
+ ratelimit_destroy(&s->rate_limit);
      bdrv_release_dirty_bitmap(s->copy_bitmap);
      shres_destroy(s->mem);
      g_free(s);
@@ -289,6 +290,7 @@ BlockCopyState *block_copy_state_new(BdrvChild *source, 
BdrvChild *target,
          s->copy_size = MAX(s->cluster_size, BLOCK_COPY_MAX_BUFFER);
      }
+ ratelimit_init(&s->rate_limit);
      QLIST_INIT(&s->tasks);
      QLIST_INIT(&s->calls);
diff --git a/blockjob.c b/blockjob.c
index 207e8c7fd9..46f15befe8 100644
--- a/blockjob.c
+++ b/blockjob.c
@@ -87,6 +87,7 @@ void block_job_free(Job *job)
block_job_remove_all_bdrv(bjob);
      blk_unref(bjob->blk);
+    ratelimit_destroy(&bjob->limit);
      error_free(bjob->blocker);
  }
@@ -435,6 +436,8 @@ void *block_job_create(const char *job_id, const BlockJobDriver *driver,
      assert(job->job.driver->free == &block_job_free);
      assert(job->job.driver->user_resume == &block_job_user_resume);
+ ratelimit_init(&job->limit);
+
      job->blk = blk;
job->finalize_cancelled_notifier.notify = block_job_event_cancelled;
diff --git a/include/qemu/ratelimit.h b/include/qemu/ratelimit.h
index 01da8d63f1..003ea6d5a3 100644
--- a/include/qemu/ratelimit.h
+++ b/include/qemu/ratelimit.h
@@ -14,9 +14,11 @@
  #ifndef QEMU_RATELIMIT_H
  #define QEMU_RATELIMIT_H
+#include "qemu/lockable.h"
  #include "qemu/timer.h"
typedef struct {
+    QemuMutex lock;
      int64_t slice_start_time;
      int64_t slice_end_time;
      uint64_t slice_quota;
@@ -40,6 +42,7 @@ static inline int64_t ratelimit_calculate_delay(RateLimit 
*limit, uint64_t n)
      int64_t now = qemu_clock_get_ns(QEMU_CLOCK_REALTIME);
      double delay_slices;
+ QEMU_LOCK_GUARD(&limit->lock);
      assert(limit->slice_quota && limit->slice_ns);
if (limit->slice_end_time < now) {
@@ -65,9 +68,20 @@ static inline int64_t ratelimit_calculate_delay(RateLimit 
*limit, uint64_t n)
      return limit->slice_end_time - now;
  }
+static inline void ratelimit_init(RateLimit *limit)
+{
+    qemu_mutex_init(&limit->lock);
+}
+
+static inline void ratelimit_destroy(RateLimit *limit)
+{
+    qemu_mutex_destroy(&limit->lock);
+}
+
  static inline void ratelimit_set_speed(RateLimit *limit, uint64_t speed,
                                         uint64_t slice_ns)
  {
+    QEMU_LOCK_GUARD(&limit->lock);
      limit->slice_ns = slice_ns;
      limit->slice_quota = MAX(((double)speed * slice_ns) / 1000000000ULL, 1);
  }


Reviewed-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>




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