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Subject[PATCH V4] xen/privcmd: improve-performance-of-mapping-of-guest
Date
One comment asked for more details on the improvements:
Using a small test program to map Guest memory into Dom0 (repeatedly
for "Iterations" mapping the same first "Num Pages")
Iterations Num Pages Time 3.7rc4 Time With this patch
5000 4096 76.107 37.027
10000 2048 75.703 37.177
20000 1024 75.893 37.247
So a little better than twice as fast.

Using this patch in migration, using "time" to measure the overall
time it take to migrate a guest (Debian Squeeze 6.0, 1024MB guest
memory, one network card, one disk, guest at login prompt):
Time 3.7rc5 Time With this patch
6.697 5.667
Since migration involves a whole lot of other things, it's only about
15% faster - but still a good improvement. Similar measurement with a
guest that is running code to "dirty" memory shows about 23%
improvement, as it spends more time copying dirtied memory.

As discussed elsewhere, a good deal more can be had from improving the
munmap system call, but it is a little tricky to get this in without
worsening non-PVOPS kernel, so I will have another look at this.

---
Update since last posting:
I have just run some benchmarks of a 16GB guest, and the improvement
with this patch is around 23-30% for the overall copy time, and 42%
shorter downtime on a "busy" guest (writing in a loop to about 8GB of
memory). I think this is definitely worth having.

The V4 patch consists of cosmetic adjustments (spelling mistake in
comment and reversing condition in an if-statement to avoid having an
"else" branch, a random empty line added by accident now reverted back
to previous state). Thanks to Jan Beulich for the comments.

The V3 of the patch contains suggested improvements from:
David Vrabel - make it two distinct external functions, doc-comments.
Ian Campbell - use one common function for the main work.
Jan Beulich - found a bug and pointed out some whitespace problems.



Signed-off-by: Mats Petersson <mats.petersson@citrix.com>

---
arch/x86/xen/mmu.c | 132 +++++++++++++++++++++++++++++++++++++++++++------
drivers/xen/privcmd.c | 55 +++++++++++++++++----
include/xen/xen-ops.h | 5 ++
3 files changed, 169 insertions(+), 23 deletions(-)

diff --git a/arch/x86/xen/mmu.c b/arch/x86/xen/mmu.c
index dcf5f2d..a67774f 100644
--- a/arch/x86/xen/mmu.c
+++ b/arch/x86/xen/mmu.c
@@ -2477,7 +2477,8 @@ void __init xen_hvm_init_mmu_ops(void)
#define REMAP_BATCH_SIZE 16

struct remap_data {
- unsigned long mfn;
+ unsigned long *mfn;
+ bool contiguous;
pgprot_t prot;
struct mmu_update *mmu_update;
};
@@ -2486,7 +2487,13 @@ static int remap_area_mfn_pte_fn(pte_t *ptep, pgtable_t token,
unsigned long addr, void *data)
{
struct remap_data *rmd = data;
- pte_t pte = pte_mkspecial(pfn_pte(rmd->mfn++, rmd->prot));
+ pte_t pte = pte_mkspecial(pfn_pte(*rmd->mfn, rmd->prot));
+ /* If we have a contigious range, just update the mfn itself,
+ else update pointer to be "next mfn". */
+ if (rmd->contiguous)
+ (*rmd->mfn)++;
+ else
+ rmd->mfn++;

rmd->mmu_update->ptr = virt_to_machine(ptep).maddr;
rmd->mmu_update->val = pte_val_ma(pte);
@@ -2495,16 +2502,34 @@ static int remap_area_mfn_pte_fn(pte_t *ptep, pgtable_t token,
return 0;
}

-int xen_remap_domain_mfn_range(struct vm_area_struct *vma,
- unsigned long addr,
- unsigned long mfn, int nr,
- pgprot_t prot, unsigned domid)
-{
+/* do_remap_mfn() - helper function to map foreign pages
+ * @vma: the vma for the pages to be mapped into
+ * @addr: the address at which to map the pages
+ * @mfn: pointer to array of MFNs to map
+ * @nr: the number entries in the MFN array
+ * @err_ptr: pointer to array
+ * @prot: page protection mask
+ * @domid: id of the domain that we are mapping from
+ *
+ * This function takes an array of mfns and maps nr pages from that into
+ * this kernel's memory. The owner of the pages is defined by domid. Where the
+ * pages are mapped is determined by addr, and vma is used for "accounting" of
+ * the pages.
+ *
+ * Return value is zero for success, negative for failure.
+ *
+ * Note that err_ptr is used to indicate whether *mfn
+ * is a list or a "first mfn of a contiguous range". */
+static int do_remap_mfn(struct vm_area_struct *vma,
+ unsigned long addr,
+ unsigned long *mfn, int nr,
+ int *err_ptr, pgprot_t prot,
+ unsigned domid)
+{
+ int err, last_err = 0;
struct remap_data rmd;
struct mmu_update mmu_update[REMAP_BATCH_SIZE];
- int batch;
unsigned long range;
- int err = 0;

if (xen_feature(XENFEAT_auto_translated_physmap))
return -EINVAL;
@@ -2515,9 +2540,15 @@ int xen_remap_domain_mfn_range(struct vm_area_struct *vma,

rmd.mfn = mfn;
rmd.prot = prot;
+ /* We use the err_ptr to indicate if there we are doing a contigious
+ * mapping or a discontigious mapping. */
+ rmd.contiguous = !err_ptr;

while (nr) {
- batch = min(REMAP_BATCH_SIZE, nr);
+ int index = 0;
+ int done = 0;
+ int batch = min(REMAP_BATCH_SIZE, nr);
+ int batch_left = batch;
range = (unsigned long)batch << PAGE_SHIFT;

rmd.mmu_update = mmu_update;
@@ -2526,19 +2557,92 @@ int xen_remap_domain_mfn_range(struct vm_area_struct *vma,
if (err)
goto out;

- err = HYPERVISOR_mmu_update(mmu_update, batch, NULL, domid);
- if (err < 0)
- goto out;
+ /* We record the error for each page that gives an error, but
+ * continue mapping until the whole set is done */
+ do {
+ err = HYPERVISOR_mmu_update(&mmu_update[index],
+ batch_left, &done, domid);
+ if (err < 0) {
+ if (!err_ptr)
+ goto out;
+ /* increment done so we skip the error item */
+ done++;
+ last_err = err_ptr[index] = err;
+ }
+ batch_left -= done;
+ index += done;
+ } while (batch_left);

nr -= batch;
addr += range;
+ if (err_ptr)
+ err_ptr += batch;
}

- err = 0;
+ err = last_err;
out:

xen_flush_tlb_all();

return err;
}
+
+/* xen_remap_domain_mfn_range() - Used to map foreign pages
+ * @vma: the vma for the pages to be mapped into
+ * @addr: the address at which to map the pages
+ * @mfn: the first MFN to map
+ * @nr: the number of consecutive mfns to map
+ * @prot: page protection mask
+ * @domid: id of the domain that we are mapping from
+ *
+ * This function takes a mfn and maps nr pages on from that into this kernel's
+ * memory. The owner of the pages is defined by domid. Where the pages are
+ * mapped is determined by addr, and vma is used for "accounting" of the
+ * pages.
+ *
+ * Return value is zero for success, negative ERRNO value for failure.
+ */
+int xen_remap_domain_mfn_range(struct vm_area_struct *vma,
+ unsigned long addr,
+ unsigned long mfn, int nr,
+ pgprot_t prot, unsigned domid)
+{
+ return do_remap_mfn(vma, addr, &mfn, nr, NULL, prot, domid);
+}
EXPORT_SYMBOL_GPL(xen_remap_domain_mfn_range);
+
+/* xen_remap_domain_mfn_array() - Used to map an array of foreign pages
+ * @vma: the vma for the pages to be mapped into
+ * @addr: the address at which to map the pages
+ * @mfn: pointer to array of MFNs to map
+ * @nr: the number entries in the MFN array
+ * @err_ptr: pointer to array of integers, one per MFN, for an error
+ * value for each page. The err_ptr must not be NULL.
+ * @prot: page protection mask
+ * @domid: id of the domain that we are mapping from
+ *
+ * This function takes an array of mfns and maps nr pages from that into this
+ * kernel's memory. The owner of the pages is defined by domid. Where the pages
+ * are mapped is determined by addr, and vma is used for "accounting" of the
+ * pages. The err_ptr array is filled in on any page that is not sucessfully
+ * mapped in.
+ *
+ * Return value is zero for success, negative ERRNO value for failure.
+ * Note that the error value -ENOENT is considered a "retry", so when this
+ * error code is seen, another call should be made with the list of pages that
+ * are marked as -ENOENT in the err_ptr array.
+ */
+int xen_remap_domain_mfn_array(struct vm_area_struct *vma,
+ unsigned long addr,
+ unsigned long *mfn, int nr,
+ int *err_ptr, pgprot_t prot,
+ unsigned domid)
+{
+ /* We BUG_ON because it's a programmer error to pass a NULL err_ptr,
+ * and the consequences later is quite hard to detect what the actual
+ * cause of "wrong memory was mapped in".
+ */
+ BUG_ON(err_ptr == NULL);
+ return do_remap_mfn(vma, addr, mfn, nr, err_ptr, prot, domid);
+}
+EXPORT_SYMBOL_GPL(xen_remap_domain_mfn_array);
diff --git a/drivers/xen/privcmd.c b/drivers/xen/privcmd.c
index 71f5c45..75f6e86 100644
--- a/drivers/xen/privcmd.c
+++ b/drivers/xen/privcmd.c
@@ -151,6 +151,40 @@ static int traverse_pages(unsigned nelem, size_t size,
return ret;
}

+/*
+ * Similar to traverse_pages, but use each page as a "block" of
+ * data to be processed as one unit.
+ */
+static int traverse_pages_block(unsigned nelem, size_t size,
+ struct list_head *pos,
+ int (*fn)(void *data, int nr, void *state),
+ void *state)
+{
+ void *pagedata;
+ unsigned pageidx;
+ int ret = 0;
+
+ BUG_ON(size > PAGE_SIZE);
+
+ pageidx = PAGE_SIZE;
+
+ while (nelem) {
+ int nr = (PAGE_SIZE/size);
+ struct page *page;
+ if (nr > nelem)
+ nr = nelem;
+ pos = pos->next;
+ page = list_entry(pos, struct page, lru);
+ pagedata = page_address(page);
+ ret = (*fn)(pagedata, nr, state);
+ if (ret)
+ break;
+ nelem -= nr;
+ }
+
+ return ret;
+}
+
struct mmap_mfn_state {
unsigned long va;
struct vm_area_struct *vma;
@@ -250,7 +284,7 @@ struct mmap_batch_state {
* 0 for no errors
* 1 if at least one error has happened (and no
* -ENOENT errors have happened)
- * -ENOENT if at least 1 -ENOENT has happened.
+ * -ENOENT if at least one -ENOENT has happened.
*/
int global_error;
/* An array for individual errors */
@@ -260,17 +294,20 @@ struct mmap_batch_state {
xen_pfn_t __user *user_mfn;
};

-static int mmap_batch_fn(void *data, void *state)
+static int mmap_batch_fn(void *data, int nr, void *state)
{
xen_pfn_t *mfnp = data;
struct mmap_batch_state *st = state;
int ret;

- ret = xen_remap_domain_mfn_range(st->vma, st->va & PAGE_MASK, *mfnp, 1,
- st->vma->vm_page_prot, st->domain);
+ BUG_ON(nr < 0);

- /* Store error code for second pass. */
- *(st->err++) = ret;
+ ret = xen_remap_domain_mfn_array(st->vma,
+ st->va & PAGE_MASK,
+ mfnp, nr,
+ st->err,
+ st->vma->vm_page_prot,
+ st->domain);

/* And see if it affects the global_error. */
if (ret < 0) {
@@ -282,7 +319,7 @@ static int mmap_batch_fn(void *data, void *state)
st->global_error = 1;
}
}
- st->va += PAGE_SIZE;
+ st->va += PAGE_SIZE * nr;

return 0;
}
@@ -378,8 +415,8 @@ static long privcmd_ioctl_mmap_batch(void __user *udata, int version)
state.err = err_array;

/* mmap_batch_fn guarantees ret == 0 */
- BUG_ON(traverse_pages(m.num, sizeof(xen_pfn_t),
- &pagelist, mmap_batch_fn, &state));
+ BUG_ON(traverse_pages_block(m.num, sizeof(xen_pfn_t),
+ &pagelist, mmap_batch_fn, &state));

up_write(&mm->mmap_sem);

diff --git a/include/xen/xen-ops.h b/include/xen/xen-ops.h
index 6a198e4..22cad75 100644
--- a/include/xen/xen-ops.h
+++ b/include/xen/xen-ops.h
@@ -29,4 +29,9 @@ int xen_remap_domain_mfn_range(struct vm_area_struct *vma,
unsigned long mfn, int nr,
pgprot_t prot, unsigned domid);

+int xen_remap_domain_mfn_array(struct vm_area_struct *vma,
+ unsigned long addr,
+ unsigned long *mfn, int nr,
+ int *err_ptr, pgprot_t prot,
+ unsigned domid);
#endif /* INCLUDE_XEN_OPS_H */
--
1.7.9.5


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