Re: [PATCH v5 09/12] contrib/plugins/hotblocks: migrate to new per_vcpu API

[Luc Michel <luc.michel@amd.com>]

On 15:09 Tue 27 Feb     , Pierrick Bouvier wrote:
> On 2/27/24 2:54 PM, Luc Michel wrote:
> > Hi Pierrick,
> > 
> > On 13:14 Mon 26 Feb     , Pierrick Bouvier wrote:
> > > Signed-off-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
> > > ---
> > >   contrib/plugins/hotblocks.c | 50 ++++++++++++++++++++++---------------
> > >   1 file changed, 30 insertions(+), 20 deletions(-)
> > > 
> > > diff --git a/contrib/plugins/hotblocks.c b/contrib/plugins/hotblocks.c
> > > index 4de1b134944..02bc5078bdd 100644
> > > --- a/contrib/plugins/hotblocks.c
> > > +++ b/contrib/plugins/hotblocks.c
> > > @@ -34,8 +34,8 @@ static guint64 limit = 20;
> > >    */
> > >   typedef struct {
> > >       uint64_t start_addr;
> > > -    uint64_t exec_count;
> > > -    int      trans_count;
> > > +    struct qemu_plugin_scoreboard *exec_count;
> > > +    int trans_count;
> > >       unsigned long insns;
> > >   } ExecCount;
> > > 
> > > @@ -43,7 +43,17 @@ static gint cmp_exec_count(gconstpointer a, gconstpointer b)
> > >   {
> > >       ExecCount *ea = (ExecCount *) a;
> > >       ExecCount *eb = (ExecCount *) b;
> > > -    return ea->exec_count > eb->exec_count ? -1 : 1;
> > > +    uint64_t count_a =
> > > +        qemu_plugin_u64_sum(qemu_plugin_scoreboard_u64(ea->exec_count));
> > > +    uint64_t count_b =
> > > +        qemu_plugin_u64_sum(qemu_plugin_scoreboard_u64(eb->exec_count));
> > > +    return count_a > count_b ? -1 : 1;
> > > +}
> > > +
> > > +static void exec_count_free(gpointer key, gpointer value, gpointer user_data)
> > > +{
> > > +    ExecCount *cnt = value;
> > > +    qemu_plugin_scoreboard_free(cnt->exec_count);
> > >   }
> > > 
> > >   static void plugin_exit(qemu_plugin_id_t id, void *p)
> > > @@ -52,7 +62,6 @@ static void plugin_exit(qemu_plugin_id_t id, void *p)
> > >       GList *counts, *it;
> > >       int i;
> > > 
> > > -    g_mutex_lock(&lock);
> > 
> > I encountered cases before where the vCPUs continue executing while
> > plugin_exit is called. This can happen e.g., when QEMU calls exit(3)
> > from one CPU thread. Others will continue to run at the same time the
> > atexit callbacks are called.
> > 
> > This also means that you can't really free the resources as you do at
> > the end of plugin_exit.
> > 
> 
> Interesting...
> 
> The current documentation [1] mentions it's the right place to free
> resources, and from what I saw, existing plugins assume this too (see
> contrib/plugins/cache.c for instance).
> 
> There is probably a bug related to the case you mention, and I'll try to
> reproduce this, and see if we can have a proper fix.
> 
> I'm not sure that removing cleanup code from existing plugins is the
> right thing to do at the moment, even though there is an existing issue
> with some programs.

Yes absolutely. The problem is on the QEMU side. FWIW I tried to fix one
of those exit cases (semihosted exit syscall) some time ago:
https://lore.kernel.org/qemu-devel/20220621125916.25257-1-lmichel@kalray.eu/

IIRC Peter was not fundamentally against it. But then I was off for
several months due to medical reasons.

I can take some time to rebase it and address Peter's comments if there
is interest to it.

Luc

> 
> [1]
> https://www.qemu.org/docs/master/devel/tcg-plugins.html#c.qemu_plugin_register_atexit_cb
> 
> > >       g_string_append_printf(report, "%d entries in the hash table\n",
> > >                              g_hash_table_size(hotblocks));
> > >       counts = g_hash_table_get_values(hotblocks);
> > > @@ -63,16 +72,21 @@ static void plugin_exit(qemu_plugin_id_t id, void *p)
> > > 
> > >           for (i = 0; i < limit && it->next; i++, it = it->next) {
> > >               ExecCount *rec = (ExecCount *) it->data;
> > > -            g_string_append_printf(report, "0x%016"PRIx64", %d, %ld, %"PRId64"\n",
> > > -                                   rec->start_addr, rec->trans_count,
> > > -                                   rec->insns, rec->exec_count);
> > > +            g_string_append_printf(
> > > +                report, "0x%016"PRIx64", %d, %ld, %"PRId64"\n",
> > > +                rec->start_addr, rec->trans_count,
> > > +                rec->insns,
> > > +                qemu_plugin_u64_sum(
> > > +                    qemu_plugin_scoreboard_u64(rec->exec_count)));
> > >           }
> > > 
> > >           g_list_free(it);
> > >       }
> > > -    g_mutex_unlock(&lock);
> > > 
> > >       qemu_plugin_outs(report->str);
> > > +
> > > +    g_hash_table_foreach(hotblocks, exec_count_free, NULL);
> > > +    g_hash_table_destroy(hotblocks);
> > >   }
> > > 
> > >   static void plugin_init(void)
> > > @@ -82,15 +96,9 @@ static void plugin_init(void)
> > > 
> > >   static void vcpu_tb_exec(unsigned int cpu_index, void *udata)
> > >   {
> > > -    ExecCount *cnt;
> > > -    uint64_t hash = (uint64_t) udata;
> > > -
> > > -    g_mutex_lock(&lock);
> > > -    cnt = (ExecCount *) g_hash_table_lookup(hotblocks, (gconstpointer) hash);
> > > -    /* should always succeed */
> > > -    g_assert(cnt);
> > > -    cnt->exec_count++;
> > > -    g_mutex_unlock(&lock);
> > > +    ExecCount *cnt = (ExecCount *)udata;
> > > +    qemu_plugin_u64_add(qemu_plugin_scoreboard_u64(cnt->exec_count),
> > > +                        cpu_index, 1);
> > >   }
> > > 
> > >   /*
> > > @@ -114,18 +122,20 @@ static void vcpu_tb_trans(qemu_plugin_id_t id, struct qemu_plugin_tb *tb)
> > >           cnt->start_addr = pc;
> > >           cnt->trans_count = 1;
> > >           cnt->insns = insns;
> > > +        cnt->exec_count = qemu_plugin_scoreboard_new(sizeof(uint64_t));
> > >           g_hash_table_insert(hotblocks, (gpointer) hash, (gpointer) cnt);
> > >       }
> > > 
> > >       g_mutex_unlock(&lock);
> > > 
> > >       if (do_inline) {
> > > -        qemu_plugin_register_vcpu_tb_exec_inline(tb, QEMU_PLUGIN_INLINE_ADD_U64,
> > > -                                                 &cnt->exec_count, 1);
> > > +        qemu_plugin_register_vcpu_tb_exec_inline_per_vcpu(
> > > +            tb, QEMU_PLUGIN_INLINE_ADD_U64,
> > > +            qemu_plugin_scoreboard_u64(cnt->exec_count), 1);
> > >       } else {
> > >           qemu_plugin_register_vcpu_tb_exec_cb(tb, vcpu_tb_exec,
> > >                                                QEMU_PLUGIN_CB_NO_REGS,
> > > -                                             (void *)hash);
> > > +                                             (void *)cnt);
> > >       }
> > >   }
> > > 
> > > --
> > > 2.43.0
> > > 
> > > 
> > 

--