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Date: Mon, 26 May 2014 15:55:27 +0800
From: Weijie Yang <weijie.yang.kh@...il.com>
To: Minchan Kim <minchan@...nel.org>,
Andrew Morton <akpm@...ux-foundation.org>
Cc: Weijie Yang <weijie.yang@...sung.com>,
Nitin Gupta <ngupta@...are.org>,
Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
Bob Liu <bob.liu@...cle.com>,
Dan Streetman <ddstreet@...e.org>,
Heesub Shin <heesub.shin@...sung.com>,
Davidlohr Bueso <davidlohr@...com>,
Joonsoo Kim <js1304@...il.com>,
linux-kernel <linux-kernel@...r.kernel.org>,
Linux-MM <linux-mm@...ck.org>
Subject: Re: [PATCH v2] zram: remove global tb_lock with fine grain lock
Hello,
Sorry for my late reply, because of a biz trip.
On Wed, May 21, 2014 at 3:51 PM, Minchan Kim <minchan@...nel.org> wrote:
> Hello Andrew,
>
> On Tue, May 20, 2014 at 03:10:51PM -0700, Andrew Morton wrote:
>> On Thu, 15 May 2014 16:00:47 +0800 Weijie Yang <weijie.yang@...sung.com> wrote:
>>
>> > Currently, we use a rwlock tb_lock to protect concurrent access to
>> > the whole zram meta table. However, according to the actual access model,
>> > there is only a small chance for upper user to access the same table[index],
>> > so the current lock granularity is too big.
>> >
>> > The idea of optimization is to change the lock granularity from whole
>> > meta table to per table entry (table -> table[index]), so that we can
>> > protect concurrent access to the same table[index], meanwhile allow
>> > the maximum concurrency.
>> > With this in mind, several kinds of locks which could be used as a
>> > per-entry lock were tested and compared:
>> >
>> > ...
>> >
>> > --- a/drivers/block/zram/zram_drv.c
>> > +++ b/drivers/block/zram/zram_drv.c
>> > @@ -179,23 +179,32 @@ static ssize_t comp_algorithm_store(struct device *dev,
>> > return len;
>> > }
>> >
>> > -/* flag operations needs meta->tb_lock */
>> > -static int zram_test_flag(struct zram_meta *meta, u32 index,
>> > - enum zram_pageflags flag)
>> > +static int zram_test_zero(struct zram_meta *meta, u32 index)
>> > {
>> > - return meta->table[index].flags & BIT(flag);
>> > + return meta->table[index].value & BIT(ZRAM_ZERO);
>> > }
>> >
>> > -static void zram_set_flag(struct zram_meta *meta, u32 index,
>> > - enum zram_pageflags flag)
>> > +static void zram_set_zero(struct zram_meta *meta, u32 index)
>> > {
>> > - meta->table[index].flags |= BIT(flag);
>> > + meta->table[index].value |= BIT(ZRAM_ZERO);
>> > }
>> >
>> > -static void zram_clear_flag(struct zram_meta *meta, u32 index,
>> > - enum zram_pageflags flag)
>> > +static void zram_clear_zero(struct zram_meta *meta, u32 index)
>> > {
>> > - meta->table[index].flags &= ~BIT(flag);
>> > + meta->table[index].value &= ~BIT(ZRAM_ZERO);
>> > +}
>> > +
>> > +static int zram_get_obj_size(struct zram_meta *meta, u32 index)
>> > +{
>> > + return meta->table[index].value & (BIT(ZRAM_FLAG_SHIFT) - 1);
>> > +}
>> > +
>> > +static void zram_set_obj_size(struct zram_meta *meta,
>> > + u32 index, int size)
>> > +{
>> > + meta->table[index].value = (unsigned long)size |
>> > + ((meta->table[index].value >> ZRAM_FLAG_SHIFT)
>> > + << ZRAM_FLAG_SHIFT );
>> > }
>>
>> Let's sort out the types here? It makes no sense for `size' to be
>> signed. And I don't think we need *any* 64-bit quantities here
>> (discussed below).
>>
>> So I think we can make `size' a u32 and remove that typecast.
>>
>> Also, please use checkpatch ;)
>>
I will remove the typecast and do checkpatch in the next patch version.
>> > static inline int is_partial_io(struct bio_vec *bvec)
>> > @@ -255,7 +264,6 @@ static struct zram_meta *zram_meta_alloc(u64 disksize)
>> > goto free_table;
>> > }
>> >
>> > - rwlock_init(&meta->tb_lock);
>> > return meta;
>> >
>> > free_table:
>> > @@ -304,19 +312,19 @@ static void handle_zero_page(struct bio_vec *bvec)
>> > flush_dcache_page(page);
>> > }
>> >
>> > -/* NOTE: caller should hold meta->tb_lock with write-side */
>>
>> Can we please update this important comment rather than simply deleting
>> it?
>>
Of couse, I will update it.
>> > static void zram_free_page(struct zram *zram, size_t index)
>> > {
>> > struct zram_meta *meta = zram->meta;
>> > unsigned long handle = meta->table[index].handle;
>> > + int size;
>> >
>> > if (unlikely(!handle)) {
>> > /*
>> > * No memory is allocated for zero filled pages.
>> > * Simply clear zero page flag.
>> > */
>> > - if (zram_test_flag(meta, index, ZRAM_ZERO)) {
>> > - zram_clear_flag(meta, index, ZRAM_ZERO);
>> > + if (zram_test_zero(meta, index)) {
>> > + zram_clear_zero(meta, index);
>> > atomic64_dec(&zram->stats.zero_pages);
>> > }
>> > return;
>> >
>> > ...
>> >
>> > @@ -64,9 +76,8 @@ enum zram_pageflags {
>> > /* Allocated for each disk page */
>> > struct table {
>> > unsigned long handle;
>> > - u16 size; /* object size (excluding header) */
>> > - u8 flags;
>> > -} __aligned(4);
>> > + unsigned long value;
>> > +};
>>
>> Does `value' need to be 64 bit on 64-bit machines? I think u32 will be
>> sufficient? The struct will still be 16 bytes but if we then play
>> around adding __packed to this structure we should be able to shrink it
>> to 12 bytes, save large amounts of memory?
>>
I agree that u32 is sufficient to value(size and flags), the reason I choice
unsigned long is as you said bit_spin_lock() requires a ulong *.
>> And does `handle' need to be 64-bit on 64-bit?
>
> To me, it's a buggy. We should not have used (unsigned long) as zsmalloc's
> handle from the beginning. Sometime it might be bigger than sizeof(unsigned long)
> because zsmalloc's handle consists of (pfn, obj idx) so pfn itself is already
> unsigned long but more practically, if we consider MAX_PHYSMEM_BITS of arch
> and zsmalloc's min size class we have some room for obj_idx which is offset
> from each pages(I think that's why it isn't a problem for CONFIG_X86_32 PAE)
> but MAX_PHYSMEM_BITS is really arch dependent thing and zsmalloc's class size
> could be changed in future so we can't make sure in (exisiting/upcoming)
> all architecture, (MAX_PHYSMEM_BITS + bit for obj_idx) is less than
> unsigned long. So we should use zs_handle rather than unsigned log and
> zs_handle's size shouldn't expose to user. :(
>
> So, I'm fine with Weijie's patch other than naming Andrew pointed out.
> I like size_and_flags. :)
>
Andrew proposed a pack idea to save more memory, when I go through it,
I think I am not convinced to use it, because:
1. it doesn't help on 32-bit system, while most embedded system are 32-bit.
2. it make code messy and unreadable.
3. it will help on 64-bit system only if "handle" can be 32-bit, but I
am not sure it.
Minchan said it's better to hide "handle" size to user, if it becomes
true, it will
be more messy for the upper pack code.
So, I like to insist this v2 patch design on the table entry.
>>
>>
>> Problem is, if we make optimisations such as this we will smash head-on
>> into the bit_spin_lock() requirement that it operate on a ulong*.
>> Which is due to the bitops requiring a ulong*. How irritating.
>>
>>
>> um, something like
>>
>> union table { /* Should be called table_entry */
>> unsigned long ul;
>> struct {
>> u32 size_and_flags;
>> u32 handle;
>> } s;
>> };
>>
>> That's a 64-bit structure containing 32-bit handle and 8-bit flags and
>> 24-bit size.
>>
>> I'm tempted to use bitfields here but that could get messy as we handle
>> endianness.
>>
>> static void zram_table_lock(union table *table)
>> {
>> #ifdef __LITTLE_ENDIAN
>> bit_spin_lock(ZRAM_ACCESS, &t->ul);
>> #else
>> #ifdef CONFIG_64BIT
>> bit_spin_lock(ZRAM_ACCESS ^ (3 << 3), &t->ul);
>> #else
>> bit_spin_lock(ZRAM_ACCESS ^ (7 << 3), &t->ul);
>> #endif
>> #endif
>> }
>>
>> Or something like that ;) And I don't know if it's correct to use
>> 32-bit handle on 64-bit.
>>
>> But you get the idea. It's worth spending time over this because the
>> space savings will be quite large.
>>
>> > struct zram_stats {
>> > atomic64_t compr_data_size; /* compressed size of pages stored */
>> >
>> > ...
>> >
>>
>> --
>> To unsubscribe, send a message with 'unsubscribe linux-mm' in
>> the body to majordomo@...ck.org. For more info on Linux MM,
>> see: http://www.linux-mm.org/ .
>> Don't email: <a href=mailto:"dont@...ck.org"> email@...ck.org </a>
>
> --
> Kind regards,
> Minchan Kim
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