Re: [RFC PATCH 2/2] NFSD: Add asynchronous write throttling support

[Christoph Hellwig <hch@lst.de>]

On Fri, Dec 19, 2025 at 09:11:05AM -0500, Chuck Lever wrote:
> From: Chuck Lever <chuck.lever@oracle.com>
> 
> When memory pressure occurs during buffered writes, the traditional
> approach is for balance_dirty_pages() to put the writing thread to
> sleep until dirty pages are flushed. For NFSD, this means server
> threads block waiting for I/O, reducing overall server throughput.
> 
> Add support for asynchronous write throttling using the BDP_ASYNC
> flag to balance_dirty_pages_ratelimited_flags(). When enabled via:
> 
>   /sys/kernel/debug/nfsd/write_async_throttle

Let me reiterate that I really, really hate all this magic debugs-fs
enabled features.  Either they are gnuinely useful (think this would
be such a thing) and they should be enabled unconditionally, or they
are tradeoffs and should have a proper tunable not hidden in debugfs.

> NFSD checks memory pressure before attempting buffered writes. If
> balance_dirty_pages_ratelimited_flags() returns -EAGAIN (indicating
> memory exhaustion), NFSD returns NFS4ERR_DELAY (or NFSERR_JUKEBOX for
> NFSv3) to the client instead of blocking.
> 
> This allows clients to back off and retry rather than having server
> threads tied up waiting for writeback. The setting defaults to 0
> (synchronous throttling) and can be combined with write_throttle for
> layered throttling strategies.
> 
> Note: NFSv2 does not support NFSERR_JUKEBOX, so async throttling is
> automatically disabled for NFSv2 requests regardless of the setting.

This all seems very useful to me.  But it really needs to show numbers
on how it helps.

> + * Contents:
> + *   %0: Synchronous throttling (default) - writes sleep in balance_dirty_pages()

Overly lone line.