[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
Re: [Xen-devel] [PATCH v2 2/3] Introduce migration precopy policy
On 20/09/17 11:20, Roger Pau Monné wrote:
On Tue, Sep 19, 2017 at 07:06:26PM +0100, Jennifer Herbert wrote:
This Patch allows a migration precopy policy to be specified.
The precopy phase of the xc_domain_save() live migration algorithm has
historically been implemented to run until either a) (almost) no pages
are dirty or b) some fixed, hard-coded maximum number of precopy
iterations has been exceeded. This policy and its implementation are
less than ideal for a few reasons:
- the logic of the policy is intertwined with the control flow of the
mechanism of the precopy stage
- it can't take into account facts external to the immediate
migration context, such external state transfer state, interactive
user input, or the passage of wall-clock time.
- it does not permit the user to change their mind, over time, about
what to do at the end of the precopy (they get an unconditional
transition into the stop-and-copy phase of the migration)
To permit callers to implement arbitrary higher-level policies governing
when the live migration precopy phase should end, and what should be
done next:
- add a precopy_policy() callback to the xc_domain_save() user-supplied
callbacks
- during the precopy phase of live migrations, consult this policy after
each batch of pages transmitted and take the dictated action, which
may be to a) abort the migration entirely, b) continue with the
precopy, or c) proceed to the stop-and-copy phase.
- provide an implementation of the old policy, used when
precopy_policy callback is not provided.
Signed-off-by: Jennifer Herbert <Jennifer.Herbert@xxxxxxxxxx>
Signed-off-by: Joshua Otto <jtotto@xxxxxxxxxxxx>
---
v2:
Have made a few formatting corrections, added typedef as suggested.
v1:
This is updated/modified subset of patch 7/20, part of
Joshua Otto's "Add postcopy live migration support." patch,
dated 27th March 2017. As indicated on the original thread,
I wish to make use of this this within the XenServer product.
I hope this will aid Josh in pushing the remainder of his series.
---
tools/libxc/include/xenguest.h | 31 ++++++++++++--
tools/libxc/xc_sr_common.h | 6 +--
tools/libxc/xc_sr_save.c | 97 +++++++++++++++++++++++++++++-------------
3 files changed, 97 insertions(+), 37 deletions(-)
diff --git a/tools/libxc/include/xenguest.h b/tools/libxc/include/xenguest.h
index 6626f0c..a2a654c 100644
--- a/tools/libxc/include/xenguest.h
+++ b/tools/libxc/include/xenguest.h
@@ -39,6 +39,16 @@
*/
struct xenevtchn_handle;
+/* For save's precopy_policy(). */
+struct precopy_stats
+{
+ unsigned int iteration;
+ unsigned int total_written;
+ long dirty_count; /* -1 if unknown */
+};
+
+typedef int (*precopy_policy_t)(struct precopy_stats, void *);
Shouldn't precopy_stats be a pointer (const pointer probably seeing
it's usage)?
In April Joshua described how he did it like this to help with IPC
plumbing into libxl.
Ian Jackson explained that you can't pass a pointer across the IPC
boundary. The two
bits of code run in different processes, with different address spaces.
Since the precopy_stats structure is tiny, it was concluded it would
have very small impact on performance.
I'd also agree since a pointer to this structure would almost half as
big at the structure itself.
I think I'll add a comment above the line to explain the decision.
+
/* callbacks provided by xc_domain_save */
struct save_callbacks {
/* Called after expiration of checkpoint interval,
@@ -46,7 +56,20 @@ struct save_callbacks {
*/
int (*suspend)(void* data);
- /* Called after the guest's dirty pages have been
+ /*
+ * Called after every batch of page data sent during the precopy
+ * phase of a live migration to ask the caller what to do next
+ * based on the current state of the precopy migration.
I would add:
"Should return one of the values listed below:"
+ */
+#define XGS_POLICY_ABORT (-1) /* Abandon the migration entirely
+ * and tidy up. */
+#define XGS_POLICY_CONTINUE_PRECOPY 0 /* Remain in the precopy phase. */
+#define XGS_POLICY_STOP_AND_COPY 1 /* Immediately suspend and transmit the
+ * remaining dirty pages. */
+ precopy_policy_t precopy_policy;
+
+ /*
+ * Called after the guest's dirty pages have been
* copied into an output buffer.
* Callback function resumes the guest & the device model,
* returns to xc_domain_save.
@@ -55,7 +78,8 @@ struct save_callbacks {
*/
int (*postcopy)(void* data);
- /* Called after the memory checkpoint has been flushed
+ /*
+ * Called after the memory checkpoint has been flushed
* out into the network. Typical actions performed in this
* callback include:
* (a) send the saved device model state (for HVM guests),
@@ -65,7 +89,8 @@ struct save_callbacks {
*
* returns:
* 0: terminate checkpointing gracefully
- * 1: take another checkpoint */
+ * 1: take another checkpoint
^ trailing space
+ */
int (*checkpoint)(void* data);
/*
diff --git a/tools/libxc/xc_sr_common.h b/tools/libxc/xc_sr_common.h
index a83f22a..3635704 100644
--- a/tools/libxc/xc_sr_common.h
+++ b/tools/libxc/xc_sr_common.h
@@ -198,12 +198,10 @@ struct xc_sr_context
/* Further debugging information in the stream. */
bool debug;
- /* Parameters for tweaking live migration. */
- unsigned max_iterations;
- unsigned dirty_threshold;
-
unsigned long p2m_size;
+ struct precopy_stats stats;
+
xen_pfn_t *batch_pfns;
unsigned nr_batch_pfns;
unsigned long *deferred_pages;
diff --git a/tools/libxc/xc_sr_save.c b/tools/libxc/xc_sr_save.c
index 1e7502d..f58c008 100644
--- a/tools/libxc/xc_sr_save.c
+++ b/tools/libxc/xc_sr_save.c
@@ -452,8 +452,7 @@ static int update_progress_string(struct xc_sr_context *ctx,
xc_interface *xch = ctx->xch;
char *new_str = NULL;
- if ( asprintf(&new_str, "Frames iteration %u of %u",
- iter, ctx->save.max_iterations) == -1 )
+ if ( asprintf(&new_str, "Frames iteration %u", iter) == -1 )
Why you still need iter here? You could easily fetch it from
ctx->save.stats.iteration
Good spot - will remove.
{
PERROR("Unable to allocate new progress string");
return -1;
@@ -467,6 +466,24 @@ static int update_progress_string(struct xc_sr_context
*ctx,
}
/*
+ * This is the live migration precopy policy - it's called periodically during
+ * the precopy phase of live migrations, and is responsible for deciding when
+ * the precopy phase should terminate and what should be done next.
+ *
+ * The policy implemented here behaves identically to the policy previously
+ * hard-coded into xc_domain_save() - it proceeds to the stop-and-copy phase of
+ * the live migration when there are either fewer than 50 dirty pages, or more
+ * than 5 precopy rounds have completed.
+ */
+static int simple_precopy_policy(struct precopy_stats stats, void *user)
+{
+ return ((stats.dirty_count >= 0 && stats.dirty_count < 50) ||
+ stats.iteration >= 5)
While here, could you make those values defines?
sure
+ ? XGS_POLICY_STOP_AND_COPY
+ : XGS_POLICY_CONTINUE_PRECOPY;
+}
+
+/*
* Send memory while guest is running.
*/
static int send_memory_live(struct xc_sr_context *ctx)
@@ -474,21 +491,58 @@ static int send_memory_live(struct xc_sr_context *ctx)
xc_interface *xch = ctx->xch;
xc_shadow_op_stats_t stats = { 0, ctx->save.p2m_size };
char *progress_str = NULL;
- unsigned x;
+ unsigned int x = 0;
int rc;
+ int policy_decision;
+
+ DECLARE_HYPERCALL_BUFFER_SHADOW(unsigned long, dirty_bitmap,
+ &ctx->save.dirty_bitmap_hbuf);
+
+ precopy_policy_t precopy_policy = ctx->save.callbacks->precopy_policy;
+ void *data = ctx->save.callbacks->data;
+
+ struct precopy_stats *policy_stats;
rc = update_progress_string(ctx, &progress_str, 0);
if ( rc )
goto out;
- rc = send_all_pages(ctx);
- if ( rc )
- goto out;
+ ctx->save.stats = (struct precopy_stats)
+ { .dirty_count = ctx->save.p2m_size };
This is exactly the same as 'stats' at this point. I'm slightly
confused about why you need 2 different stats variable, plus a pointer
to a stats variable (stats, ctx->save.stats and *policy_stats).
They do start off similar, and are certainly closely related.
xc_shadow_op_stats_t stats has different fields in it then precopy_stats
policy_stats.
The former has a fault and dirty count, per iteration, while the latter has
iteration number, total_written (over all iterations) and dirty count.
*policy_stats is just a convenience pointer, reducing the amount of
indirection on
every access. I though this made it easier to read.
+ policy_stats = &ctx->save.stats;
+
+ if ( precopy_policy == NULL )
+ precopy_policy = simple_precopy_policy;
+
+ bitmap_set(dirty_bitmap, ctx->save.p2m_size);
+
+ do {
+ policy_decision = precopy_policy(*policy_stats, data);
The comment at the top says:
"Called after every batch of page data sent during the precopy phase"
Yet here the hook seems to be called before any processing has been
done for the first iteration of the loop.
I'll change to "Called before and after every batch ...."
+ x++;
Also updating x here seems weird, we completely ignore iteration 0.
The line above the 'x++' checks the policy using 'iteration 0'. In
patch v1 I used the x variable in initialising the stats, to try and
suggest this, but as its zero, and the default value for a struct is
zero, it was concluded that was unnecessary. In any case,
logically, this is where it moves from one 'iteration' to another.
Previously there was no iteration zero, as it started on zero.
Now iteration zero is to indicate the starting state.
Combining this comment with Paul's, it could use:
for (x = 1; ; ++x)
If this is thought to be more readable - although Andrew cooper
described a loop looking like this as "suspicious" on Joshua's version
of this patch.
I have no strong feelings on the matter.... let me know.
+
+ if ( stats.dirty_count > 0 && policy_decision != XGS_POLICY_ABORT )
Trailing space at the end of the line.
+ {
+ rc = update_progress_string(ctx, &progress_str, x);
+ if ( rc )
+ goto out;
+
+ rc = send_dirty_pages(ctx, stats.dirty_count);
+ if ( rc )
+ goto out;
+ }
+
+ if ( policy_decision != XGS_POLICY_CONTINUE_PRECOPY )
+ break;
+
+ policy_stats->iteration = x;
+ policy_stats->total_written += policy_stats->dirty_count;
+ policy_stats->dirty_count = -1;
+
+ policy_decision = precopy_policy(*policy_stats, data);
+
+ if ( policy_decision != XGS_POLICY_CONTINUE_PRECOPY )
+ break;
- for ( x = 1;
- ((x < ctx->save.max_iterations) &&
- (stats.dirty_count > ctx->save.dirty_threshold)); ++x )
- {
if ( xc_shadow_control(
xch, ctx->domid, XEN_DOMCTL_SHADOW_OP_CLEAN,
&ctx->save.dirty_bitmap_hbuf, ctx->save.p2m_size,
@@ -499,17 +553,9 @@ static int send_memory_live(struct xc_sr_context *ctx)
goto out;
}
- if ( stats.dirty_count == 0 )
- break;
+ policy_stats->dirty_count = stats.dirty_count;
- rc = update_progress_string(ctx, &progress_str, x);
- if ( rc )
- goto out;
-
- rc = send_dirty_pages(ctx, stats.dirty_count);
- if ( rc )
- goto out;
- }
+ } while ( true );
out:
xc_set_progress_prefix(xch, NULL);
@@ -601,7 +647,7 @@ static int suspend_and_send_dirty(struct xc_sr_context *ctx)
if ( ctx->save.live )
{
rc = update_progress_string(ctx, &progress_str,
- ctx->save.max_iterations);
+ ctx->save.stats.iteration);
Hm, this as mentioned above seems redundant (ctx is already a
parameter of update_progress_string.
Thanks, Roger.
Thanks for the feedback,
-jenny
_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
https://lists.xen.org/xen-devel
|
