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[PATCH v2] ring-buffer: Align meta-page to sub-buffers for improved TLB usage

Vincent Donnefort posted 1 patch 1 year, 5 months ago
[PATCH v2] ring-buffer: Align meta-page to sub-buffers for improved TLB usage
Posted by Vincent Donnefort 1 year, 5 months ago 
Previously, the mapped ring-buffer layout caused misalignment between
the meta-page and sub-buffers when the sub-buffer size was not a
multiple of PAGE_SIZE. This prevented hardware with larger TLB entries
from utilizing them effectively.

Add a padding with the zero-page between the meta-page and sub-buffers.
Also update the ring-buffer map_test to verify that padding.

Signed-off-by: Vincent Donnefort <vdonnefort@google.com>

--

This is based on the mm-unstable branch [1] as it depends on David's work [2]
for allowing the zero-page in vm_insert_page().

[1] https://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm.git
[2] https://lore.kernel.org/all/20240522125713.775114-1-david@redhat.com

v1 -> v2:
  * Fix unsequenced modification and access to 'p' (s390 build)


diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 7345a8b625fb..c1116e76fe17 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -6148,10 +6148,10 @@ static void rb_setup_ids_meta_page(struct ring_buffer_per_cpu *cpu_buffer,
 	/* install subbuf ID to kern VA translation */
 	cpu_buffer->subbuf_ids = subbuf_ids;
 
-	meta->meta_page_size = PAGE_SIZE;
 	meta->meta_struct_len = sizeof(*meta);
 	meta->nr_subbufs = nr_subbufs;
 	meta->subbuf_size = cpu_buffer->buffer->subbuf_size + BUF_PAGE_HDR_SIZE;
+	meta->meta_page_size = meta->subbuf_size;
 
 	rb_update_meta_page(cpu_buffer);
 }
@@ -6238,6 +6238,12 @@ static int __rb_map_vma(struct ring_buffer_per_cpu *cpu_buffer,
 	    !(vma->vm_flags & VM_MAYSHARE))
 		return -EPERM;
 
+	subbuf_order = cpu_buffer->buffer->subbuf_order;
+	subbuf_pages = 1 << subbuf_order;
+
+	if (subbuf_order && pgoff % subbuf_pages)
+		return -EINVAL;
+
 	/*
 	 * Make sure the mapping cannot become writable later. Also tell the VM
 	 * to not touch these pages (VM_DONTCOPY | VM_DONTEXPAND).
@@ -6247,11 +6253,8 @@ static int __rb_map_vma(struct ring_buffer_per_cpu *cpu_buffer,
 
 	lockdep_assert_held(&cpu_buffer->mapping_lock);
 
-	subbuf_order = cpu_buffer->buffer->subbuf_order;
-	subbuf_pages = 1 << subbuf_order;
-
 	nr_subbufs = cpu_buffer->nr_pages + 1; /* + reader-subbuf */
-	nr_pages = ((nr_subbufs) << subbuf_order) - pgoff + 1; /* + meta-page */
+	nr_pages = ((nr_subbufs + 1) << subbuf_order) - pgoff; /* + meta-page */
 
 	vma_pages = (vma->vm_end - vma->vm_start) >> PAGE_SHIFT;
 	if (!vma_pages || vma_pages > nr_pages)
@@ -6264,20 +6267,24 @@ static int __rb_map_vma(struct ring_buffer_per_cpu *cpu_buffer,
 		return -ENOMEM;
 
 	if (!pgoff) {
+		unsigned long meta_page_padding;
+
 		pages[p++] = virt_to_page(cpu_buffer->meta_page);
 
 		/*
-		 * TODO: Align sub-buffers on their size, once
-		 * vm_insert_pages() supports the zero-page.
+		 * Pad with the zero-page to align the meta-page with the
+		 * sub-buffers.
 		 */
-	} else {
-		/* Skip the meta-page */
-		pgoff--;
+		meta_page_padding = subbuf_pages - 1;
+		while (meta_page_padding-- && p < nr_pages) {
+			unsigned long __maybe_unused zero_addr =
+				vma->vm_start + (PAGE_SIZE * p);
 
-		if (pgoff % subbuf_pages) {
-			err = -EINVAL;
-			goto out;
+			pages[p++] = ZERO_PAGE(zero_addr);
 		}
+	} else {
+		/* Skip the meta-page */
+		pgoff -= subbuf_pages;
 
 		s += pgoff / subbuf_pages;
 	}
diff --git a/tools/testing/selftests/ring-buffer/map_test.c b/tools/testing/selftests/ring-buffer/map_test.c
index a9006fa7097e..4bb0192e43f3 100644
--- a/tools/testing/selftests/ring-buffer/map_test.c
+++ b/tools/testing/selftests/ring-buffer/map_test.c
@@ -228,6 +228,20 @@ TEST_F(map, data_mmap)
 	data = mmap(NULL, data_len, PROT_READ, MAP_SHARED,
 		    desc->cpu_fd, meta_len);
 	ASSERT_EQ(data, MAP_FAILED);
+
+	/* Verify meta-page padding */
+	if (desc->meta->meta_page_size > getpagesize()) {
+		void *addr;
+
+		data_len = desc->meta->meta_page_size;
+		data = mmap(NULL, data_len,
+			    PROT_READ, MAP_SHARED, desc->cpu_fd, 0);
+		ASSERT_NE(data, MAP_FAILED);
+
+		addr = (void *)((unsigned long)data + getpagesize());
+		ASSERT_EQ(*((int *)addr), 0);
+		munmap(data, data_len);
+	}
 }
 
 FIXTURE(snapshot) {

base-commit: c65920c76a977c2b73c3a8b03b4c0c00cc1285ed
-- 
2.45.2.803.g4e1b14247a-goog
Re: [PATCH v2] ring-buffer: Align meta-page to sub-buffers for improved TLB usage
Posted by Steven Rostedt 1 year, 3 months ago 
On Fri, 28 Jun 2024 11:46:11 +0100
Vincent Donnefort <vdonnefort@google.com> wrote:

> diff --git a/tools/testing/selftests/ring-buffer/map_test.c b/tools/testing/selftests/ring-buffer/map_test.c
> index a9006fa7097e..4bb0192e43f3 100644
> --- a/tools/testing/selftests/ring-buffer/map_test.c
> +++ b/tools/testing/selftests/ring-buffer/map_test.c
> @@ -228,6 +228,20 @@ TEST_F(map, data_mmap)
>  	data = mmap(NULL, data_len, PROT_READ, MAP_SHARED,
>  		    desc->cpu_fd, meta_len);
>  	ASSERT_EQ(data, MAP_FAILED);
> +
> +	/* Verify meta-page padding */
> +	if (desc->meta->meta_page_size > getpagesize()) {
> +		void *addr;
> +
> +		data_len = desc->meta->meta_page_size;
> +		data = mmap(NULL, data_len,
> +			    PROT_READ, MAP_SHARED, desc->cpu_fd, 0);
> +		ASSERT_NE(data, MAP_FAILED);
> +
> +		addr = (void *)((unsigned long)data + getpagesize());
> +		ASSERT_EQ(*((int *)addr), 0);

Should we make this a test that the entire page is zero?

		for (int i = desc->meta->meta_struct_len; i < desc->meta->meta_page_size; i += sizeof(int))
			ASSERT_EQ(((int *)data)[i], 0);

?

> +		munmap(data, data_len);
> +	}
>  }

Also, looking at the init, if for some reason (I highly doubt it may
happen) that the meta_struct_len becomes bigger than page_size, we should
update the init section to:

	/* Handle the case where meta_struct_len is greater than page size */
	if (page_size < desc->meta->meta_struct_len) {
		/* meta_page_size is >= meta_struct_len */
		page_size = desc->meta->meta_page_size;
		munmap(desc->meta, page_size);
		map = mmap(NULL, page_size, PROT_READ, MAP_SHARED, desc->cpu_fd, 0);
		if (map == MAP_FAILED)
			return -errno;
		desc->meta = (struct trace_buffer_meta *)map;
	}

-- Steve
[PATCH 1/2] ring-buffer/selftest: Verify the entire meta-page padding
Posted by Vincent Donnefort 1 year, 3 months ago 
Improve the ring-buffer meta-page test coverage by checking for the
entire padding region to be 0 instead of just looking at the first 4
bytes.

Signed-off-by: Vincent Donnefort <vdonnefort@google.com>

--

Hi,

I saw you have sent "Align meta-page to sub-buffers for improved TLB usage" to
linux-next, so here's a follow-up patch addressing your comments, not sure if
you want to squash it or to put it on top.

diff --git a/tools/testing/selftests/ring-buffer/map_test.c b/tools/testing/selftests/ring-buffer/map_test.c
index 4bb0192e43f3..ba12fd31de87 100644
--- a/tools/testing/selftests/ring-buffer/map_test.c
+++ b/tools/testing/selftests/ring-buffer/map_test.c
@@ -231,15 +231,15 @@ TEST_F(map, data_mmap)
 
 	/* Verify meta-page padding */
 	if (desc->meta->meta_page_size > getpagesize()) {
-		void *addr;
-
 		data_len = desc->meta->meta_page_size;
 		data = mmap(NULL, data_len,
 			    PROT_READ, MAP_SHARED, desc->cpu_fd, 0);
 		ASSERT_NE(data, MAP_FAILED);
 
-		addr = (void *)((unsigned long)data + getpagesize());
-		ASSERT_EQ(*((int *)addr), 0);
+		for (int i = desc->meta->meta_struct_len;
+		     i < desc->meta->meta_page_size; i += sizeof(int))
+			ASSERT_EQ(*(int *)(data + i), 0);
+
 		munmap(data, data_len);
 	}
 }

base-commit: 2a07e30c19f391af26517c409fd66e401c6f4ee7
prerequisite-patch-id: 16b79d676c5faf3b57443b576976c7522fcd5a4b
-- 
2.46.0.295.g3b9ea8a38a-goog
[PATCH 2/2] ring-buffer/selftest: Handle meta-page bigger than the system
Posted by Vincent Donnefort 1 year, 3 months ago 
Handle the case where the meta-page content is bigger than the system
page-size. This prepares the ground for extending features covered by
the meta-page.

Signed-off-by: Vincent Donnefort <vdonnefort@google.com>

diff --git a/tools/testing/selftests/ring-buffer/map_test.c b/tools/testing/selftests/ring-buffer/map_test.c
index ba12fd31de87..d10a847130fb 100644
--- a/tools/testing/selftests/ring-buffer/map_test.c
+++ b/tools/testing/selftests/ring-buffer/map_test.c
@@ -92,12 +92,22 @@ int tracefs_cpu_map(struct tracefs_cpu_map_desc *desc, int cpu)
 	if (desc->cpu_fd < 0)
 		return -ENODEV;
 
+again:
 	map = mmap(NULL, page_size, PROT_READ, MAP_SHARED, desc->cpu_fd, 0);
 	if (map == MAP_FAILED)
 		return -errno;
 
 	desc->meta = (struct trace_buffer_meta *)map;
 
+	/* the meta-page is bigger than the original mapping */
+	if (page_size < desc->meta->meta_struct_len) {
+		int meta_page_size = desc->meta->meta_page_size;
+
+		munmap(desc->meta, page_size);
+		page_size = meta_page_size;
+		goto again;
+	}
+
 	return 0;
 }
 
-- 
2.46.0.295.g3b9ea8a38a-goog
Re: [PATCH v2] ring-buffer: Align meta-page to sub-buffers for improved TLB usage
Posted by Steven Rostedt 1 year, 5 months ago 
On Fri, 28 Jun 2024 11:46:11 +0100
Vincent Donnefort <vdonnefort@google.com> wrote:

> This is based on the mm-unstable branch [1] as it depends on David's work [2]
> for allowing the zero-page in vm_insert_page().
> 
> [1] https://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm.git
> [2] https://lore.kernel.org/all/20240522125713.775114-1-david@redhat.com

I'll hold off to 6.12 merge window before pushing this. So that the
above will be guaranteed to be there.

-- Steve

Patchew 2.1-devel-b67e4c2

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