Re: [Qemu-devel] [PATCH] migration: Fix multi-thread compression bug

[Li, Liang Z]

> To: Li, Liang Z
> Cc: address@hidden; address@hidden; address@hidden
> Subject: Re: [Qemu-devel] [PATCH] migration: Fix multi-thread compression
> bug
> 
> Liang Li <address@hidden> wrote:
> > Recently, a bug related to multiple thread compression feature for
> > live migration is reported. The destination side will be blocked
> > during live migration if there are heavy workload in host and memory
> > intensive workload in guest, this is most likely to happen when there
> > is one decompression thread.
> >
> > Some parts of the decompression code are incorrect:
> > 1. The main thread receives data from source side will enter a busy
> > loop to wait for a free decompression thread.
> > 2. A lock is needed to protect the decomp_param[idx]->start, because
> > it is checked in the main thread and is updated in the decompression
> > thread.
> >
> > Fix these two issues by following the code pattern for compression.
> >
> > Reported-by: Daniel P. Berrange <address@hidden>
> > Signed-off-by: Liang Li <address@hidden>
> 
> step in the right direction, so:
> Reviewed-by: Juan Quintela <address@hidden>
> 
> but I am still not sure that this is
> enough.  if you have the change, look at the multiple-fd code that I posted, 
> is
> very similar here.
> 
> 
> >  struct DecompressParam {
> 
> what protect start, and what protect done?
> 

decomp_param[i]-> mutex protects start and  decomp_done_lock
protects done.

> 
> >      bool start;
> > +    bool done;
> >      QemuMutex mutex;
> >      QemuCond cond;
> >      void *des;
> > @@ -287,6 +288,8 @@ static bool quit_comp_thread;  static bool
> > quit_decomp_thread;  static DecompressParam *decomp_param;  static
> > QemuThread *decompress_threads;
> > +static QemuMutex decomp_done_lock;
> > +static QemuCond decomp_done_cond;
> >
> >  static int do_compress_ram_page(CompressParam *param);
> >
> > @@ -834,6 +837,7 @@ static inline void
> start_compression(CompressParam
> > *param)
> >
> >  static inline void start_decompression(DecompressParam *param)  {
> 
> Here nothing protects done

start_decompression is call when holding the decomp_done_lock.

> 
> > +    param->done = false;
> >      qemu_mutex_lock(&param->mutex);
> >      param->start = true;
> >      qemu_cond_signal(&param->cond);
> > @@ -2193,19 +2197,24 @@ static void *do_data_decompress(void
> *opaque)
> >          qemu_mutex_lock(&param->mutex);
> 
> we are looking at quit_decomp_thread and nothing protects it
> 
> 
> >          while (!param->start && !quit_decomp_thread) {
> >              qemu_cond_wait(&param->cond, &param->mutex);
> > +        }
> > +        if (!quit_decomp_thread) {
> >              pagesize = TARGET_PAGE_SIZE;
> > -            if (!quit_decomp_thread) {
> > -                /* uncompress() will return failed in some case, especially
> > -                 * when the page is dirted when doing the compression, it's
> > -                 * not a problem because the dirty page will be 
> > retransferred
> > -                 * and uncompress() won't break the data in other pages.
> > -                 */
> > -                uncompress((Bytef *)param->des, &pagesize,
> > -                           (const Bytef *)param->compbuf, param->len);
> > -            }
> > -            param->start = false;
> > +            /* uncompress() will return failed in some case, especially
> > +             * when the page is dirted when doing the compression, it's
> > +             * not a problem because the dirty page will be retransferred
> > +             * and uncompress() won't break the data in other pages.
> > +             */
> > +            uncompress((Bytef *)param->des, &pagesize,
> > +                       (const Bytef *)param->compbuf, param->len);
> 
> We are calling uncompress (a slow operation) with param->mutex taken, is
> there any reason why we can't just put the param->* vars in locals?
> 
> >          }
> > +        param->start = false;
> 
> Why are we setting start to false when we _are_ not decompressing a page?
> I think this line should be inside the loop.
> 
> >          qemu_mutex_unlock(&param->mutex);
> > +
> > +        qemu_mutex_lock(&decomp_done_lock);
> > +        param->done = true;
> 
> here param->done is protected by decomp_done_lock.
> 
> > +        qemu_cond_signal(&decomp_done_cond);
> > +        qemu_mutex_unlock(&decomp_done_lock);
> >      }
> >
> >      return NULL;
> > @@ -2219,10 +2228,13 @@ void
> migrate_decompress_threads_create(void)
> >      decompress_threads = g_new0(QemuThread, thread_count);
> >      decomp_param = g_new0(DecompressParam, thread_count);
> >      quit_decomp_thread = false;
> > +    qemu_mutex_init(&decomp_done_lock);
> > +    qemu_cond_init(&decomp_done_cond);
> >      for (i = 0; i < thread_count; i++) {
> >          qemu_mutex_init(&decomp_param[i].mutex);
> >          qemu_cond_init(&decomp_param[i].cond);
> >          decomp_param[i].compbuf =
> > g_malloc0(compressBound(TARGET_PAGE_SIZE));
> > +        decomp_param[i].done = true;
> >          qemu_thread_create(decompress_threads + i, "decompress",
> >                             do_data_decompress, decomp_param + i,
> >                             QEMU_THREAD_JOINABLE); @@ -2258,9 +2270,10
> > @@ static void decompress_data_with_multi_threads(QEMUFile *f,
> >      int idx, thread_count;
> >
> >      thread_count = migrate_decompress_threads();
> > +    qemu_mutex_lock(&decomp_done_lock);
> 
> we took decomp_done_lock
> 
> >      while (true) {
> >          for (idx = 0; idx < thread_count; idx++) {
> > -            if (!decomp_param[idx].start) {
> > +            if (decomp_param[idx].done) {
> 
> and we can protecet done with it.
> 
> >                  qemu_get_buffer(f, decomp_param[idx].compbuf, len);
> >                  decomp_param[idx].des = host;
> >                  decomp_param[idx].len = len;
> 
> but this ones should be proteced by docomp_param[idx].mutex, no?

The code can work correct, but it looks confusion, it seems I should make the 
lock more clear.
I will try to change it by referencing you multi -fd code. Thanks!


Liang

> 
> > @@ -2270,8 +2283,11 @@ static void
> decompress_data_with_multi_threads(QEMUFile *f,
> >          }
> >          if (idx < thread_count) {
> >              break;
> > +        } else {
> > +            qemu_cond_wait(&decomp_done_cond, &decomp_done_lock);
> >          }
> >      }
> > +    qemu_mutex_unlock(&decomp_done_lock);
> >  }
> >
> >  /*
> 
> Thanks, Juan.