Re: [PATCH] multipath-tools: improve processing efficiency for addition and deletion of multipath devices

[Martin Wilck <mwilck@suse.com>]

Hello Tang,

I'm sorry to reply so late. Thanks a lot for your work, I agree with
Ben that the patch is in pretty good shape now. But I have some
remarks left, please see below. 
 
> +bool
> +uevent_can_discard(struct uevent *uev)
> +{
> +	char *tmp;
> +	char a[11], b[11];
> +	struct config * conf;
> +
> +	/*
> +	 * keep only block devices, discard partitions
> +	 */
> +	tmp = strstr(uev->devpath, "/block/");
> +	if (tmp == NULL){
> +		condlog(4, "no /block/ in '%s'", uev->devpath);
> +		return true;
> +	}
> +	if (sscanf(tmp, "/block/%10s", a) != 1 ||
> +	    sscanf(tmp, "/block/%10[^/]/%10s", a, b) == 2) {
> +		condlog(4, "discard event on %s", uev->devpath);
> +		return true;
> +	}

I'd like the following better for this check. It uses much less cycles.

static bool
can_discard_by_devpath(const char *devpath)
{
	static const char BLOCK[] = "/block/";
	const char *p;

	p = strstr(pathstr, BLOCK);
	if (p == NULL)
		/* not a block device */
		return true;
	p += sizeof(BLOCK) - 1;
	p = strchr(p, '/');
	if (p == NULL)
		/* exactly one path element after "/block/" */
		return false;
	/* If there are more path elements, it's a partition */
	return true;
}

> +bool
> +uevent_can_filter(struct uevent *earlier, struct uevent *later)
> +{
> +
> +	/*
> +	 * filter earlier uvents if path has removed later. Eg:
> +	 * "add path1 |chang path1 |add path2 |remove path1"
> +	 * can filter as:
> +	 * "add path2 |remove path1"
> +	 * uevents "add path1" and "chang path1" are filtered out
> +	 */
> +	if (!strcmp(earlier->kernel, later->kernel) &&
> +		!strcmp(later->action, "remove") &&
> +		strncmp(later->kernel, "dm-", 3)) {
> +		return true;
> +	}
> +
> +	/*
> +	 * filter change uvents if add uevents exist. Eg:
> +	 * "change path1| add path1 |add path2"
> +	 * can filter as:
> +	 * "add path1 |add path2"
> +	 * uevent "chang path1" is filtered out
> +	 */
> +	if (!strcmp(earlier->kernel, later->kernel) &&
> +		!strcmp(earlier->action, "change") &&
> +		!strcmp(later->action, "add") &&
> +		strncmp(later->kernel, "dm-", 3)) {
> +		return true;
> +	}
> +
> +	return false;
> +}

This would be better readable and faster if you'd put the "kernel"
tests first so that you need to check only "action" later:

	if (!strncmp(later->kernel, "dm-", 3) ||
	    strcmp(earlier->kernel, later->kernel))
		return false;

> +
> +bool
> +merge_need_stop(struct uevent *earlier, struct uevent *later)
> +{
> +	/*
> +	 * dm uevent do not try to merge with left uevents
> +	 */
> +	if (!strncmp(later->kernel, "dm-", 3))
> +		return true;
> +
> +	/*
> +	 * we can not make a jugement without wwid,
> +	 * so it is sensible to stop merging
> +	 */
> +	if (!earlier->wwid || !later->wwid)
> +		return true;
> +	/*
> +	 * uevents merging stoped
> +	 * when we meet an opposite action uevent from the same LUN
> to AVOID
> +	 * "add path1 |remove path1 |add path2 |remove path2 |add
> path3"
> +	 * to merge as "remove path1, path2" and "add path1, path2,
> path3"
> +	 * OR
> +	 * "remove path1 |add path1 |remove path2 |add path2 |remove
> path3"
> +	 * to merge as "add path1, path2" and "remove path1, path2,
> path3"
> +	 * SO
> +	 * when we meet a non-change uevent from the same LUN
> +	 * with the same wwid and different action
> +	 * it would be better to stop merging.
> +	 */
> +	if (!strcmp(earlier->wwid, later->wwid) &&
> +	    strcmp(earlier->action, later->action) &&
> +	    strcmp(earlier->action, "change") &&
> +	    strcmp(later->action, "change"))
> +		return true;
> +
> +	return false;
> +}

I know you discussed this with Ben before, but still have some trouble
with it. 

The first case should have been reduced to "remove path1 | remove path2
| add path3" by filtering beforehand. I suppose you want to avoid this
sequence because it could leave us without paths temporarily, causing
multipathd to destroy the map. But I don't understand what "stop
merging" buys you here - if you process the events one-by-one, you may
also have 0 paths at some point. 

In the second case, we know all events in the sequence have the same
WWID; in this case I think it would be safe to filter away "remove"
events by subsequent "add" events, ending up with "add path1| add
path2| remove path3". But I may be overlooking something here.

The dangerous thing if you have simultaneous remove and add events for
the same LUN is that processing the "add" events is likely to fail in
domap(). If you get "add path1 | remove path2", once you process "add
path1", "path2" may not exist in the kernel any more, and "domap" will
fail if you try to set up both; you may end up removing the map
completely. IMHO the only safe way to process events in this situation
is to merge the events into a single domap() call.

I know you want to avoid that in this patch, but I think it will be a
logical further improvement.

Anyway, AFAICS your patch doesn't introduce a regression wrt the
current code here; unless I'm overlooking something, my arguments would
apply to sequential event processing as well.

Regards
Martin

-- 
Dr. Martin Wilck <mwilck@suse.com>, Tel. +49 (0)911 74053 2107
SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton
HRB 21284 (AG Nürnberg)

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