Re: [PATCH v1 1/2] oslib-posix: refactor memory prealloc threads

[Mark Kanda]

On 1/9/24 8:25 AM, David Hildenbrand wrote:
On 09.01.24 15:15, Daniel P. Berrangé wrote:
On Tue, Jan 09, 2024 at 03:02:00PM +0100, David Hildenbrand wrote:
On 08.01.24 19:40, Mark Kanda wrote:
On 1/8/24 9:40 AM, David Hildenbrand wrote:
On 08.01.24 16:10, Mark Kanda wrote:
Refactor the memory prealloc threads support:
- Make memset context a global qlist
- Move the memset thread join/cleanup code to a separate routine

This is functionally equivalent and facilitates multiple memset 
contexts
(used in a subsequent patch).

Signed-off-by: Mark Kanda <mark.ka...@oracle.com>
---
    util/oslib-posix.c | 104 
+++++++++++++++++++++++++++++----------------
    1 file changed, 68 insertions(+), 36 deletions(-)

diff --git a/util/oslib-posix.c b/util/oslib-posix.c
index e86fd64e09..293297ac6c 100644
--- a/util/oslib-posix.c
+++ b/util/oslib-posix.c
@@ -63,11 +63,15 @@
      struct MemsetThread;
    +static QLIST_HEAD(, MemsetContext) memset_contexts =
+    QLIST_HEAD_INITIALIZER(memset_contexts);
+
    typedef struct MemsetContext {
        bool all_threads_created;
        bool any_thread_failed;
        struct MemsetThread *threads;
        int num_threads;
+    QLIST_ENTRY(MemsetContext) next;
    } MemsetContext;
      struct MemsetThread {
@@ -81,7 +85,7 @@ struct MemsetThread {
    typedef struct MemsetThread MemsetThread;
      /* used by sigbus_handler() */
-static MemsetContext *sigbus_memset_context;
+static bool sigbus_memset_context;
    struct sigaction sigbus_oldact;
    static QemuMutex sigbus_mutex;
    @@ -295,13 +299,16 @@ static void sigbus_handler(int signal)
    #endif /* CONFIG_LINUX */
    {
        int i;
+    MemsetContext *context;
          if (sigbus_memset_context) {
-        for (i = 0; i < sigbus_memset_context->num_threads; i++) {
-            MemsetThread *thread = 
&sigbus_memset_context->threads[i];
+        QLIST_FOREACH(context, &memset_contexts, next) {
+            for (i = 0; i < context->num_threads; i++) {
+                MemsetThread *thread = &context->threads[i];
    -            if (qemu_thread_is_self(&thread->pgthread)) {
-                siglongjmp(thread->env, 1);
+                if (qemu_thread_is_self(&thread->pgthread)) {
+                    siglongjmp(thread->env, 1);
+                }
                }
            }
        }
@@ -417,14 +424,15 @@ static int touch_all_pages(char *area, size_t
hpagesize, size_t numpages,
                               bool use_madv_populate_write)
    {
        static gsize initialized = 0;
-    MemsetContext context = {
-        .num_threads = get_memset_num_threads(hpagesize, numpages,
max_threads),
-    };
+    MemsetContext *context = g_malloc0(sizeof(MemsetContext));
        size_t numpages_per_thread, leftover;
        void *(*touch_fn)(void *);
-    int ret = 0, i = 0;
+    int i = 0;
        char *addr = area;
    +    context->num_threads =
+        get_memset_num_threads(hpagesize, numpages, max_threads);
+
        if (g_once_init_enter(&initialized)) {
            qemu_mutex_init(&page_mutex);
            qemu_cond_init(&page_cond);
@@ -433,7 +441,7 @@ static int touch_all_pages(char *area, size_t
hpagesize, size_t numpages,
          if (use_madv_populate_write) {
            /* Avoid creating a single thread for 
MADV_POPULATE_WRITE */
-        if (context.num_threads == 1) {
+        if (context->num_threads == 1) {
                if (qemu_madvise(area, hpagesize * numpages,
QEMU_MADV_POPULATE_WRITE)) {
                    return -errno;
@@ -445,49 +453,74 @@ static int touch_all_pages(char *area, size_t
hpagesize, size_t numpages,
            touch_fn = do_touch_pages;
        }
    -    context.threads = g_new0(MemsetThread, 
context.num_threads);
-    numpages_per_thread = numpages / context.num_threads;
-    leftover = numpages % context.num_threads;
-    for (i = 0; i < context.num_threads; i++) {
-        context.threads[i].addr = addr;
-        context.threads[i].numpages = numpages_per_thread + (i <
leftover);
-        context.threads[i].hpagesize = hpagesize;
-        context.threads[i].context = &context;
+    context->threads = g_new0(MemsetThread, context->num_threads);
+    numpages_per_thread = numpages / context->num_threads;
+    leftover = numpages % context->num_threads;
+    for (i = 0; i < context->num_threads; i++) {
+        context->threads[i].addr = addr;
+        context->threads[i].numpages = numpages_per_thread + (i <
leftover);
+        context->threads[i].hpagesize = hpagesize;
+        context->threads[i].context = context;
            if (tc) {
-            thread_context_create_thread(tc,
&context.threads[i].pgthread,
+            thread_context_create_thread(tc,
&context->threads[i].pgthread,
"touch_pages",
-                                         touch_fn, 
&context.threads[i],
+                                         touch_fn,
&context->threads[i],
QEMU_THREAD_JOINABLE);
            } else {
- qemu_thread_create(&context.threads[i].pgthread, "touch_pages",
-                               touch_fn, &context.threads[i],
+ qemu_thread_create(&context->threads[i].pgthread, "touch_pages",
+                               touch_fn, &context->threads[i],
QEMU_THREAD_JOINABLE);
            }
-        addr += context.threads[i].numpages * hpagesize;
+        addr += context->threads[i].numpages * hpagesize;
        }
    +    QLIST_INSERT_HEAD(&memset_contexts, context, next);
+
        if (!use_madv_populate_write) {
-        sigbus_memset_context = &context;
+        sigbus_memset_context = true;

Thanks David,

Could we just use the sigbus handling alone and support parallel init
only when using MADV_POPULATE_WRITE where we don't have to mess with
signal handlers?


Ideally, we're hoping to support this with earlier kernels which don't
support MADV_POPULATE _WRITE. But, I will check to see if we really 
need it.

That's around since Linux 5.14, so please try simplifying.


Further, how do you changes interact with other callers of
qemu_prealloc_mem(), like virtio-mem?


I'm not familiar with the intricacies of virtio-mem, but the basic 
idea
of this series is to *only* allow parallel init during the start up
phase (while prealloc_init == false). Once we have parsed all the
command line args, we set prealloc_init = true
(wait_mem_prealloc_init()) which causes all subsequent calls to
qemu_prealloc_mem() to perform initialization synchronously. So, I
*think* this covers the virtio-mem use case. Am I missing something?

Good, so this should likely not affect virtio-mem (which also ends up
preallocating memory when loading from a vmstate).

To make this all a bit clearer, what about the following to make this:

* Optimize for MADV_POPULATE_WRITE. If we really need support for
   !MADV_POPULATE_WRITE, this is better added on top later.
* Pass in via a parameter that the caller requests async handling. 
"bool
   async" should be good enough. Then, pass that only from the memory
   backend call, while QEMU is still initializing (we can find a way to
   make that work).
* Provide a function that waits for any remaining async os-mem-prealloc
   activity. That is essentially "wait_mem_prealloc_init", but without
   the special internal flag handling.

Further, I do wonder if we want to make that behavior configurable. For
example, one might want to initialize backends sequentially using 16 
threads
max, instead of consuming multiple times 16 threads concurrently.

Seems to me that no matter what parallelisation we use (within
mem regions, or across mem regions, or a mix of both), we should
never use more threads than there are host CPUs.

Yes. It gets tricky with multipe NUMA nodes, though. And that's what's 
being requested here.


Can we have a pool of threads sized per available host CPUs that
QEMU can access. Then for each memory backend fire off a set of
init jobs that get distributed to "appropriate" threads in the
pool. By appropriate I mean threads with the same NUMA affinity
as the memory backend. This would give parallelisation both
within a single large memory region, as well as across memory
regions.


eg 4 host CPUs, 3 memory regions (1GB for 1st numa node, 1GB
for 2nd numa node, and 1 GB with no numa affinity). If we
spread the init work then we end up with

I'll note that having a mixture of numa + no numa is not a common 
config we should try to optimize.


  1st thread gets 500MB to init from 1st memory region, and
      250MB to init from 3rd memory region

  2st thread gets 500MB to init from 1st memory region, and
      250MB to init from 3rd memory region

  3st thread gets 500MB to init from 2nd memory region, and
      250MB to init from 3rd memory region

  4th thread gets 500MB to init from 2nd memory region, and
      250MB to init from 3rd memory region

We do have prealloc contexts that are used to craft new CPUs with the 
right NUMA affinity. Prealloc contexts are set for memory backends. So 
extending prealloc context to limit/reuse threads and do the pooling 
might be possible.


Could
either be a machine option or a memory backend option 
(async-prealloc=on).
Once hotplugging such backends, we would disable it for now, but 
could allow
it in the future.

IMHO "do the right thing" without user config is preferrable to adding
yet more cli options here.

Sure, if there is an easy way.


Thank you David and Daniel for your feedback. I plan to address your 
suggestions in v2 (including optimizing for MADV_POPULATE_WRITE).

Best regards,
-Mark