Re: [uml-devel] [PATCH 3/3] BUG: Inapprorpiate choice of IPC

[Richard Weinberger <richard@nod.at>]

Am 07.03.2014 19:37, schrieb anton.ivanov@kot-begemot.co.uk:
> From: Anton Ivanov <antivano@cisco.com>
> 
> PIPE is the wrong IPC for this type of inter-thread communication.
> 
> For more details see:
> 
> http://stackoverflow.com/questions/4624071/pipe-buffer-size-is-4k-or-64k
> 
> The observations on that thread have been confirmed by us for UML's
> UBD driver. If you load UML with network IO and do disk IO at the same
> time the UBD IO helper thread fails to process requests fast enough.
> 
> In most cases this will lead to a slowdown in disk IO as well as
> inability to execute new processes until the network load goes away.
> In some cases however it will not recover. Example - with our new high
> performance 1G+ network drivers a wget of 1G file from the network
> is a nearly guaranteed crash if it writes out to UBD.
> 
> The crashes and the slowdowns are not observed with this patch -
> it switches the IPC to socket which does not have the pipe granularity
> and queue size problems.
> 
> This signifies a problem with overall UBD error handling which this
> patch fails to fix. The workaround should be good enough for most
> cases.
> 
> Signed-off-by: Anton Ivanov <antivano@cisco.com>
> ---
>  arch/um/drivers/ubd_kern.c |    2 +-
>  arch/um/drivers/ubd_user.c |    2 +-
>  arch/um/os-Linux/file.c    |    2 +-
>  3 files changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/um/drivers/ubd_kern.c b/arch/um/drivers/ubd_kern.c
> index 944453a..c9a5717 100644
> --- a/arch/um/drivers/ubd_kern.c
> +++ b/arch/um/drivers/ubd_kern.c
> @@ -1291,7 +1291,7 @@ static void do_ubd_request(struct request_queue *q)
>  			n = os_write_file(thread_fd, &io_req,
>  					  sizeof(struct io_thread_req *));
>  			if(n != sizeof(struct io_thread_req *)){
> -				if(n != -EAGAIN)
> +				if(!((n == -EAGAIN) || (n == -ENOBUFS)))
>  					printk("write to io thread failed, "
>  					       "errno = %d\n", -n);
>  				else if(list_empty(&dev->restart))
> diff --git a/arch/um/drivers/ubd_user.c b/arch/um/drivers/ubd_user.c
> index 007b94d..f1f84a4 100644
> --- a/arch/um/drivers/ubd_user.c
> +++ b/arch/um/drivers/ubd_user.c
> @@ -32,7 +32,7 @@ int start_io_thread(unsigned long sp, int *fd_out)
>  {
>  	int pid, fds[2], err;
>  
> -	err = os_pipe(fds, 1, 1);
> +	err = socketpair(AF_UNIX, SOCK_STREAM, 0, (int *) &fds);
>  	if(err < 0){
>  		printk("start_io_thread - os_pipe failed, err = %d\n", -err);
>  		goto out;
> diff --git a/arch/um/os-Linux/file.c b/arch/um/os-Linux/file.c
> index c879b12..a4f0e65 100644
> --- a/arch/um/os-Linux/file.c
> +++ b/arch/um/os-Linux/file.c
> @@ -240,7 +240,7 @@ void os_close_file(int fd)
>  int os_fsync_file(int fd)
>  {
>  	if (fsync(fd) < 0) 
> -	    return -errno
> +	    return -errno;

Huh?
I think this does not belong to the patch.

The other drivers look good. Thanks a lot for pushing this work upstream!
We are already at -rc5. I'll send the patches to Linus within the next merge window
and mark them for stable.

Thanks,
//richard

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