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From: Filipe Manana <fdmanana@gmail.com>
To: Qu Wenruo <wqu@suse.com>
Cc: linux-btrfs <linux-btrfs@vger.kernel.org>
Subject: Re: [PATCH v2] btrfs: transaction: Commit transaction more frequently for BPF
Date: Fri, 16 Aug 2019 10:33:15 +0100
Message-ID: <CAL3q7H75=BecnH0L34OAKmQcbHPDegsO6YxVxJgg--wv_cgciA@mail.gmail.com> (raw)
In-Reply-To: <20190815080404.20600-1-wqu@suse.com>

On Thu, Aug 15, 2019 at 9:36 AM Qu Wenruo <wqu@suse.com> wrote:
>
> Btrfs has btrfs_end_transaction_throttle() which could try to commit
> transaction when needed.
>
> However under most cases btrfs_end_transaction_throttle() won't really
> commit transaction, due to the hard timing requirement.
>
> Now introduce a new error injection point, btrfs_need_trans_pressure(),
> to allow btrfs_should_end_transaction() to return 1 and
> btrfs_end_transaction_throttle() to fallback to
> btrfs_commit_transaction().
>
> With such more aggressive transaction commit, we can dig deeper into
> cases like snapshot drop.
> Now each reference drop of btrfs_drop_snapshot() will lead to a
> transaction commit, allowing dm-logwrites to catch more details, other
> than one big transaction dropping everything.
>
> Signed-off-by: Qu Wenruo <wqu@suse.com>
> ---
> Changelog:
> v2:
> - Add comment to explain why this function is needed
> ---
>  fs/btrfs/transaction.c | 18 ++++++++++++++++++
>  1 file changed, 18 insertions(+)
>
> diff --git a/fs/btrfs/transaction.c b/fs/btrfs/transaction.c
> index 3f6811cdf803..8c5471b01d03 100644
> --- a/fs/btrfs/transaction.c
> +++ b/fs/btrfs/transaction.c
> @@ -10,6 +10,7 @@
>  #include <linux/pagemap.h>
>  #include <linux/blkdev.h>
>  #include <linux/uuid.h>
> +#include <linux/error-injection.h>
>  #include "ctree.h"
>  #include "disk-io.h"
>  #include "transaction.h"
> @@ -749,10 +750,25 @@ void btrfs_throttle(struct btrfs_fs_info *fs_info)
>         wait_current_trans(fs_info);
>  }
>
> +/*
> + * This function is to allow BPF to override the return value so that we can
> + * make btrfs to commit transaction more aggressively.
> + *
> + * It's a debug only feature, mainly used with dm-log-writes to catch more details
> + * of transient operations like balance and subvolume drop.

Transient? I think you mean long running operations that can span
multiple transactions.

> + */
> +static noinline bool btrfs_need_trans_pressure(struct btrfs_trans_handle *trans)
> +{
> +       return false;
> +}
> +ALLOW_ERROR_INJECTION(btrfs_need_trans_pressure, TRUE);

So, I'm not sure if it's really a good idea to have such specific
things like this.
Has this proven useful already? I.e., have you already found any bug using this?

I often add such similar things myself for testing and debugging, but
because they are so specific, or ugly or verbose, I keep them to
myself.

Allowing the return value of should_end_transaction() to be
overridden, using the same approach, would be more generic for
example.

Thanks.

> +
>  static int should_end_transaction(struct btrfs_trans_handle *trans)
>  {
>         struct btrfs_fs_info *fs_info = trans->fs_info;
>
> +       if (btrfs_need_trans_pressure(trans))
> +               return 1;
>         if (btrfs_check_space_for_delayed_refs(fs_info))
>                 return 1;
>
> @@ -813,6 +829,8 @@ static int __btrfs_end_transaction(struct btrfs_trans_handle *trans,
>
>         btrfs_trans_release_chunk_metadata(trans);
>
> +       if (throttle && btrfs_need_trans_pressure(trans))
> +               return btrfs_commit_transaction(trans);
>         if (lock && READ_ONCE(cur_trans->state) == TRANS_STATE_BLOCKED) {
>                 if (throttle)
>                         return btrfs_commit_transaction(trans);
> --
> 2.22.0
>


-- 
Filipe David Manana,

“Whether you think you can, or you think you can't — you're right.”

  reply index

Thread overview: 8+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-08-15  8:04 Qu Wenruo
2019-08-16  9:33 ` Filipe Manana [this message]
2019-08-16  9:53   ` Qu Wenruo
2019-08-16 10:03     ` Filipe Manana
2019-08-16 10:06       ` Qu Wenruo
2019-08-19 16:57       ` David Sterba
2019-08-20  0:34         ` Qu Wenruo
2019-08-19  5:14   ` Qu Wenruo

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