Re: User mode stalls - can it be...?

[Roger Larsson <roger.larsson@norran.net>]

Roger Larsson wrote:

What about this modification (buffer.c):

 static void sync_page_buffers(struct buffer_head *bh, int wait)
{
	struct buffer_head * tmp = bh;

	do {
		struct buffer_head *p = tmp;
		tmp = tmp->b_this_page;
		if (buffer_locked(p)) {
			if (wait)
				__wait_on_buffer(p);
		} else if (buffer_dirty(p))
			ll_rw_block(WRITE, 1, &p);
	} while (tmp != bh);
}

-to-

static void sync_page_buffers(struct buffer_head *bh, int wait)
{
	struct buffer_head * tmp = bh;

	do {
		struct buffer_head *p = tmp;
		tmp = tmp->b_this_page;
		if (!buffer_locked(p) &&
                    buffer_dirty(p))
			ll_rw_block(WRITE, 1, &p);
		if (wait)
			__wait_on_buffer(p);
	} while (tmp != bh);
}


Not tested... (tomorrow - good night)

/RogerL

> 
> Hi,
> 
> I have an idea why we get the stalls!
> 
> If we end up in a situation where our write attempts are rejected
> due to lack of resources - we will continue happily with next and
> next...
> 
> Count hits zero and we return
> 
> But kswapd finds out that there is more to do and calls another
> shrink_mmap...
> (and need_resched is not set due to lower priority)
> 
> Gives, busy wait in shrink_mmap until pages are written...
> 
> I tried to renice 15 kswapd with improved results (I am using my patched
> kernels)
> Another approach would be to always sleep...
> 
> /RogerL
> 
> (Sent privately to Quintela and Riel - but it might be of value for
>  someone else too...)
> 
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