Re: [PATCH] Xen backend support for paged out grant targets.

[Andres Lagar-Cavilla <andreslc@gridcentric.ca>]

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But msleep will wind up calling schedule(). We definitely cannot afford to
pin down a dom0 vcpu when the pager itself is in dom0.

Iiuc....

Andres
On Aug 31, 2012 12:55 PM, "Konrad Rzeszutek Wilk" <konrad.wilk@oracle.com>
wrote:

> On Fri, Aug 31, 2012 at 11:42:32AM -0400, Andres Lagar-Cavilla wrote:
> > Actually acted upon your feedback ipso facto:
> >
> > commit d5fab912caa1f0cf6be0a6773f502d3417a207b6
> > Author: Andres Lagar-Cavilla <andres@lagarcavilla.org>
> > Date:   Sun Aug 26 09:45:57 2012 -0400
> >
> >     Xen backend support for paged out grant targets.
> >
> >     Since Xen-4.2, hvm domains may have portions of their memory paged
> out. When a
> >     foreign domain (such as dom0) attempts to map these frames, the map
> will
> >     initially fail. The hypervisor returns a suitable errno, and kicks an
> >     asynchronous page-in operation carried out by a helper. The foreign
> domain is
> >     expected to retry the mapping operation until it eventually
> succeeds. The
> >     foreign domain is not put to sleep because itself could be the one
> running the
> >     pager assist (typical scenario for dom0).
> >
> >     This patch adds support for this mechanism for backend drivers using
> grant
> >     mapping and copying operations. Specifically, this covers the
> blkback and
> >     gntdev drivers (which map foregin grants), and the netback driver
> (which copies
> >     foreign grants).
> >
> >     * Add GNTST_eagain, already exposed by Xen, to the grant interface.
> >     * Add a retry method for grants that fail with GNTST_eagain (i.e.
> because the
> >       target foregin frame is paged out).
> >     * Insert hooks with appropriate macro decorators in the
> aforementioned drivers.
> >
> >     The retry loop is only invoked if the grant operation status is
> GNTST_eagain.
> >     It guarantees to leave a new status code different from
> GNTST_eagain. Any other
> >     status code results in identical code execution as before.
> >
> >     The retry loop performs 256 attempts with increasing time intervals
> through a
> >     32 second period. It uses msleep to yield while waiting for the next
> retry.
> >
>
>
> Would it make sense to yield to other processes (so call schedule)? Or
> perhaps have this in a workqueue ?
>
> I mean the 'msleep' just looks like a hack. .. 32 seconds of doing
> 'msleep' on 1VCPU dom0 could trigger the watchdog I think?
>
> >     V2 after feedback from David Vrabel:
> >     * Explicit MAX_DELAY instead of wrap-around delay into zero
> >     * Abstract GNTST_eagain check into core grant table code for netback
> module.
> >
> >     Signed-off-by: Andres Lagar-Cavilla <andres@lagarcavilla.org>
> >
> > diff --git a/drivers/net/xen-netback/netback.c
> b/drivers/net/xen-netback/netback.c
> > index 682633b..5610fd8 100644
> > --- a/drivers/net/xen-netback/netback.c
> > +++ b/drivers/net/xen-netback/netback.c
> > @@ -635,9 +635,7 @@ static void xen_netbk_rx_action(struct xen_netbk
> *netbk)
> >               return;
> >
> >       BUG_ON(npo.copy_prod > ARRAY_SIZE(netbk->grant_copy_op));
> > -     ret = HYPERVISOR_grant_table_op(GNTTABOP_copy,
> &netbk->grant_copy_op,
> > -                                     npo.copy_prod);
> > -     BUG_ON(ret != 0);
> > +     gnttab_batch_copy_no_eagain(netbk->grant_copy_op, npo.copy_prod);
> >
> >       while ((skb = __skb_dequeue(&rxq)) != NULL) {
> >               sco = (struct skb_cb_overlay *)skb->cb;
> > @@ -1460,18 +1458,15 @@ static void xen_netbk_tx_submit(struct xen_netbk
> *netbk)
> >  static void xen_netbk_tx_action(struct xen_netbk *netbk)
> >  {
> >       unsigned nr_gops;
> > -     int ret;
> >
> >       nr_gops = xen_netbk_tx_build_gops(netbk);
> >
> >       if (nr_gops == 0)
> >               return;
> > -     ret = HYPERVISOR_grant_table_op(GNTTABOP_copy,
> > -                                     netbk->tx_copy_ops, nr_gops);
> > -     BUG_ON(ret);
> >
> > -     xen_netbk_tx_submit(netbk);
> > +     gnttab_batch_copy_no_eagain(netbk->tx_copy_ops, nr_gops);
> >
> > +     xen_netbk_tx_submit(netbk);
> >  }
> >
> >  static void xen_netbk_idx_release(struct xen_netbk *netbk, u16
> pending_idx)
> > diff --git a/drivers/xen/grant-table.c b/drivers/xen/grant-table.c
> > index eea81cf..96543b2 100644
> > --- a/drivers/xen/grant-table.c
> > +++ b/drivers/xen/grant-table.c
> > @@ -38,6 +38,7 @@
> >  #include <linux/vmalloc.h>
> >  #include <linux/uaccess.h>
> >  #include <linux/io.h>
> > +#include <linux/delay.h>
> >  #include <linux/hardirq.h>
> >
> >  #include <xen/xen.h>
> > @@ -823,6 +824,26 @@ unsigned int gnttab_max_grant_frames(void)
> >  }
> >  EXPORT_SYMBOL_GPL(gnttab_max_grant_frames);
> >
> > +#define MAX_DELAY 256
> > +void
> > +gnttab_retry_eagain_gop(unsigned int cmd, void *gop, int16_t *status,
> > +                                             const char *func)
> > +{
> > +     unsigned delay = 1;
> > +
> > +     do {
> > +             BUG_ON(HYPERVISOR_grant_table_op(cmd, gop, 1));
> > +             if (*status == GNTST_eagain)
> > +                     msleep(delay++);
> > +     } while ((*status == GNTST_eagain) && (delay < MAX_DELAY));
> > +
> > +     if (delay >= MAX_DELAY) {
> > +             printk(KERN_ERR "%s: %s eagain grant\n", func,
> current->comm);
> > +             *status = GNTST_bad_page;
> > +     }
> > +}
> > +EXPORT_SYMBOL_GPL(gnttab_retry_eagain_gop);
> > +
> >  int gnttab_map_refs(struct gnttab_map_grant_ref *map_ops,
> >                   struct gnttab_map_grant_ref *kmap_ops,
> >                   struct page **pages, unsigned int count)
> > @@ -836,6 +857,11 @@ int gnttab_map_refs(struct gnttab_map_grant_ref
> *map_ops,
> >       if (ret)
> >               return ret;
> >
> > +     /* Retry eagain maps */
> > +     for (i = 0; i < count; i++)
> > +             if (map_ops[i].status == GNTST_eagain)
> > +                     gnttab_retry_eagain_map(map_ops + i);
> > +
> >       if (xen_feature(XENFEAT_auto_translated_physmap))
> >               return ret;
> >
> > diff --git a/drivers/xen/xenbus/xenbus_client.c
> b/drivers/xen/xenbus/xenbus_client.c
> > index b3e146e..749f6a3 100644
> > --- a/drivers/xen/xenbus/xenbus_client.c
> > +++ b/drivers/xen/xenbus/xenbus_client.c
> > @@ -490,8 +490,7 @@ static int xenbus_map_ring_valloc_pv(struct
> xenbus_device *dev,
> >
> >       op.host_addr = arbitrary_virt_to_machine(pte).maddr;
> >
> > -     if (HYPERVISOR_grant_table_op(GNTTABOP_map_grant_ref, &op, 1))
> > -             BUG();
> > +     gnttab_map_grant_no_eagain(&op);
> >
> >       if (op.status != GNTST_okay) {
> >               free_vm_area(area);
> > @@ -572,8 +571,7 @@ int xenbus_map_ring(struct xenbus_device *dev, int
> gnt_ref,
> >       gnttab_set_map_op(&op, (unsigned long)vaddr, GNTMAP_host_map,
> gnt_ref,
> >                         dev->otherend_id);
> >
> > -     if (HYPERVISOR_grant_table_op(GNTTABOP_map_grant_ref, &op, 1))
> > -             BUG();
> > +     gnttab_map_grant_no_eagain(&op);
> >
> >       if (op.status != GNTST_okay) {
> >               xenbus_dev_fatal(dev, op.status,
> > diff --git a/include/xen/grant_table.h b/include/xen/grant_table.h
> > index 11e27c3..2fecfab 100644
> > --- a/include/xen/grant_table.h
> > +++ b/include/xen/grant_table.h
> > @@ -43,6 +43,7 @@
> >  #include <xen/interface/grant_table.h>
> >
> >  #include <asm/xen/hypervisor.h>
> > +#include <asm/xen/hypercall.h>
> >
> >  #include <xen/features.h>
> >
> > @@ -183,6 +184,43 @@ unsigned int gnttab_max_grant_frames(void);
> >
> >  #define gnttab_map_vaddr(map) ((void *)(map.host_virt_addr))
> >
> > +/* Retry a grant map/copy operation when the hypervisor returns
> GNTST_eagain.
> > + * This is typically due to paged out target frames.
> > + * Generic entry-point, use macro decorators below for specific grant
> > + * operations.
> > + * Will retry for 1, 2, ... 255 ms, i.e. 256 times during 32 seconds.
> > + * Return value in *status guaranteed to no longer be GNTST_eagain. */
> > +void gnttab_retry_eagain_gop(unsigned int cmd, void *gop, int16_t
> *status,
> > +                             const char *func);
> > +
> > +#define gnttab_retry_eagain_map(_gop)                       \
> > +    gnttab_retry_eagain_gop(GNTTABOP_map_grant_ref, (_gop), \
> > +                            &(_gop)->status, __func__)
> > +
> > +#define gnttab_retry_eagain_copy(_gop)                  \
> > +    gnttab_retry_eagain_gop(GNTTABOP_copy, (_gop),      \
> > +                            &(_gop)->status, __func__)
> > +
> > +#define gnttab_map_grant_no_eagain(_gop)
>      \
> > +do {
>      \
> > +    if (HYPERVISOR_grant_table_op(GNTTABOP_map_grant_ref, (_gop), 1))
>     \
> > +        BUG();
>      \
> > +    if ((_gop)->status == GNTST_eagain)
>     \
> > +        gnttab_retry_eagain_map((_gop));
>      \
> > +} while(0)
> > +
> > +static inline void
> > +gnttab_batch_copy_no_eagain(struct gnttab_copy *batch, unsigned count)
> > +{
> > +    unsigned i;
> > +    struct gnttab_copy *op;
> > +
> > +    BUG_ON(HYPERVISOR_grant_table_op(GNTTABOP_copy, batch, count));
> > +    for (i = 0, op = batch; i < count; i++, op++)
> > +        if (op->status == GNTST_eagain)
> > +            gnttab_retry_eagain_copy(op);
> > +}
> > +
> >  int gnttab_map_refs(struct gnttab_map_grant_ref *map_ops,
> >                   struct gnttab_map_grant_ref *kmap_ops,
> >                   struct page **pages, unsigned int count);
> > diff --git a/include/xen/interface/grant_table.h
> b/include/xen/interface/grant_table.h
> > index 7da811b..66cb734 100644
> > --- a/include/xen/interface/grant_table.h
> > +++ b/include/xen/interface/grant_table.h
> > @@ -520,6 +520,7 @@ DEFINE_GUEST_HANDLE_STRUCT(gnttab_get_version);
> >  #define GNTST_permission_denied (-8) /* Not enough privilege for
> operation.  */
> >  #define GNTST_bad_page         (-9) /* Specified page was invalid for
> op.    */
> >  #define GNTST_bad_copy_arg    (-10) /* copy arguments cross page
> boundary */
> > +#define GNTST_eagain          (-12) /* Retry.
>      */
> >
> >  #define GNTTABOP_error_msgs {                   \
> >      "okay",                                     \
> > @@ -533,6 +534,7 @@ DEFINE_GUEST_HANDLE_STRUCT(gnttab_get_version);
> >      "permission denied",                        \
> >      "bad page",                                 \
> >      "copy arguments cross page boundary"        \
> > +    "retry"                                     \
> >  }
> >
> >  #endif /* __XEN_PUBLIC_GRANT_TABLE_H__ */
> >
> >
> > On Aug 31, 2012, at 10:45 AM, Andres Lagar-Cavilla wrote:
> >
> > >
> > > On Aug 31, 2012, at 10:32 AM, David Vrabel wrote:
> > >
> > >> On 27/08/12 17:51, andres@lagarcavilla.org wrote:
> > >>> From: Andres Lagar-Cavilla <andres@lagarcavilla.org>
> > >>>
> > >>> Since Xen-4.2, hvm domains may have portions of their memory paged
> out. When a
> > >>> foreign domain (such as dom0) attempts to map these frames, the map
> will
> > >>> initially fail. The hypervisor returns a suitable errno, and kicks an
> > >>> asynchronous page-in operation carried out by a helper. The foreign
> domain is
> > >>> expected to retry the mapping operation until it eventually
> succeeds. The
> > >>> foreign domain is not put to sleep because itself could be the one
> running the
> > >>> pager assist (typical scenario for dom0).
> > >>>
> > >>> This patch adds support for this mechanism for backend drivers using
> grant
> > >>> mapping and copying operations. Specifically, this covers the
> blkback and
> > >>> gntdev drivers (which map foregin grants), and the netback driver
> (which copies
> > >>> foreign grants).
> > >>>
> > >>> * Add GNTST_eagain, already exposed by Xen, to the grant interface.
> > >>> * Add a retry method for grants that fail with GNTST_eagain (i.e.
> because the
> > >>> target foregin frame is paged out).
> > >>> * Insert hooks with appropriate macro decorators in the
> aforementioned drivers.
> > >>
> > >> I think you should implement wrappers around
> HYPERVISOR_grant_table_op()
> > >> have have the wrapper do the retries instead of every backend having
> to
> > >> check for EAGAIN and issue the retries itself. Similar to the
> > >> gnttab_map_grant_no_eagain() function you've already added.
> > >>
> > >> Why do some operations not retry anyway?
> > >
> > > All operations retry. The reason why I could not make it as elegant as
> you suggest is because grant operations are submitted in batches and their
> status(es?) later checked individually elsewhere. This is the case for
> netback. Note that both blkback and gntdev use a more linear structure with
> the gnttab_map_refs helper, which allows me to hide all the retry gore from
> those drivers into grant table code. Likewise for xenbus ring mapping.
> > >
> > > In summary, outside of core grant table code, only the netback driver
> needs to check explicitly for retries, due to its
> batch-copy-delayed-per-slot-check structure.
> > >
> > >>
> > >>> +void
> > >>> +gnttab_retry_eagain_gop(unsigned int cmd, void *gop, int16_t
> *status,
> > >>> +                                         const char *func)
> > >>> +{
> > >>> + u8 delay = 1;
> > >>> +
> > >>> + do {
> > >>> +         BUG_ON(HYPERVISOR_grant_table_op(cmd, gop, 1));
> > >>> +         if (*status == GNTST_eagain)
> > >>> +                 msleep(delay++);
> > >>> + } while ((*status == GNTST_eagain) && delay);
> > >>
> > >> Terminating the loop when delay wraps is a bit subtle.  Why not make
> > >> delay unsigned and check delay <= MAX_DELAY?
> > > Good idea (MAX_DELAY == 256). I'd like to get Konrad's feedback before
> a re-spin.
> > >
> > >>
> > >> Would it be sensible to ramp the delay faster?  Perhaps double each
> > >> iteration with a maximum possible delay of e.g., 256 ms.
> > > Generally speaking we've never seen past three retries. I am open to
> changing the algorithm but there is a significant possibility it won't
> matter at all.
> > >
> > >>
> > >>> +#define gnttab_map_grant_no_eagain(_gop)
>          \
> > >>> +do {
>          \
> > >>> +    if ( HYPERVISOR_grant_table_op(GNTTABOP_map_grant_ref, (_gop),
> 1))      \
> > >>> +        BUG();
>          \
> > >>> +    if ((_gop)->status == GNTST_eagain)
>         \
> > >>> +        gnttab_retry_eagain_map((_gop));
>          \
> > >>> +} while(0)
> > >>
> > >> Inline functions, please.
> > >
> > > I want to retain the original context for debugging. Eventually we
> print __func__ if things go wrong.
> > >
> > > Thanks, great feedback
> > > Andres
> > >
> > >>
> > >> David
> > >
> >
>

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