Re: [PATCH] tools/libxc: send page-in requests in batches in linux_privcmd_map_foreign_bulk

["Andres Lagar-Cavilla" <andres@lagarcavilla.org>]

> Date: Thu, 26 Jan 2012 15:51:02 +0100
> From: Olaf Hering <olaf@aepfle.de>
> To: xen-devel@lists.xensource.com
> Subject: [Xen-devel] [PATCH] tools/libxc: send page-in requests in
> 	batches in linux_privcmd_map_foreign_bulk
> Message-ID: <bace47d7623cb92d5b08.1327589462@probook.site>
> Content-Type: text/plain; charset="us-ascii"
>
> # HG changeset patch
> # User Olaf Hering <olaf@aepfle.de>
> # Date 1327589312 -3600
> # Node ID bace47d7623cb92d5b080c54d3850642b8c52b46
> # Parent  a87cbe503fed9e15d90d0bf6645d835331ec8874
> tools/libxc: send page-in requests in batches in
> linux_privcmd_map_foreign_bulk
>
> One of the bottlenecks with foreign page-in request is the poor retry
> handling in linux_privcmd_map_foreign_bulk(). It sends one request per
> paged gfn at a time and it waits until the gfn is accessible. This
> causes long delays in mmap requests from qemu-dm and xc_save.
>
> Instead of sending one request at a time, walk the entire gfn list and
> send batches of mmap requests. They will eventually end up in the pagers
> request ring (if it has room again), and will fill up this ring so that
> in turn the pager can also process page-in in batches.
>
> Signed-off-by: Olaf Hering <olaf@aepfle.de>
>
> diff -r a87cbe503fed -r bace47d7623c tools/libxc/xc_linux_osdep.c
> --- a/tools/libxc/xc_linux_osdep.c
> +++ b/tools/libxc/xc_linux_osdep.c
> @@ -191,6 +191,59 @@ static void *linux_privcmd_map_foreign_b
>      return addr;
>  }
>
> +/*
> + * Retry mmap of paged gfns in batches
> + * retuns < 0 on fatal error
> + * returns 0 if all gfns left paging state
> + * returns > 0 if some gfns are still in paging state
> + *
> + * Walk all gfns are assemble blocks of gfns in paging state.
> + * This will keep the request ring full and avoids delays.
> + */
> +static int retry_paged(int fd, uint32_t dom, void *addr,
> +                       const xen_pfn_t *arr, int *err, unsigned int num)
> +{
> +    privcmd_mmapbatch_v2_t ioctlx;
> +    int rc, paged = 0, i = 0;
> +
> +    do
> +    {
> +        /* Skip gfns not in paging state */
> +        if ( err[i] != -ENOENT )
> +        {
> +            i++;
> +            continue;
> +        }
> +
> +        paged++;
> +
> +        /* At least one gfn is still in paging state */
> +        ioctlx.num = 1;
> +        ioctlx.dom = dom;
> +        ioctlx.addr = (unsigned long)addr + ((unsigned
> long)i<<XC_PAGE_SHIFT);
> +        ioctlx.arr = arr + i;
> +        ioctlx.err = err + i;
> +
> +        /* Assemble a batch of requests */
> +        while ( ++i < num )
> +        {
> +            if ( err[i] != -ENOENT )
> +                break;
> +            ioctlx.num++;
> +        }

Olaf,
if I get a pattern of errors in the err array of the form:

{ENOENT, EWHATEVER, ENOENT, EWHATEVER}
with EWHATEVER != ENOENT

this patch does not optimize anything.

Why not alloc a separate arr2 array, move the gfn's that failed with
ENOENT there, and retry the whole new arr2 block? You only need to
allocate arr2 once, to size num. That will batch always, everything.

Thanks,
Andres
> +
> +        /* Send request and abort on fatal error */
> +        rc = ioctl(fd, IOCTL_PRIVCMD_MMAPBATCH_V2, &ioctlx);
> +        if ( rc < 0 && errno != ENOENT )
> +            goto out;
> +
> +    } while ( i < num );
> +
> +    rc = paged;
> +out:
> +    return rc;
> +}
> +
>  static void *linux_privcmd_map_foreign_bulk(xc_interface *xch,
> xc_osdep_handle h,
>                                              uint32_t dom, int prot,
>                                              const xen_pfn_t *arr, int
> *err, unsigned int num)
> @@ -220,21 +273,10 @@ static void *linux_privcmd_map_foreign_b
>      /* Command was recognized, some gfn in arr are in paging state */
>      if ( rc < 0 && errno == ENOENT )
>      {
> -        for ( i = rc = 0; rc == 0 && i < num; i++ )
> -        {
> -            if ( err[i] != -ENOENT )
> -                continue;
> -
> -            ioctlx.num = 1;
> -            ioctlx.dom = dom;
> -            ioctlx.addr = (unsigned long)addr + ((unsigned
> long)i<<XC_PAGE_SHIFT);
> -            ioctlx.arr = arr + i;
> -            ioctlx.err = err + i;
> -            do {
> -                usleep(100);
> -                rc = ioctl(fd, IOCTL_PRIVCMD_MMAPBATCH_V2, &ioctlx);
> -            } while ( rc < 0 && errno == ENOENT && err[i] == -ENOENT );
> -        }
> +        do {
> +            usleep(100);
> +            rc = retry_paged(fd, dom, addr, arr, err, num);
> +        } while ( rc > 0 );
>      }
>      /* Command was not recognized, use fall back */
>      else if ( rc < 0 && errno == EINVAL && (int)num > 0 )
>
>
>