Can you tap all your runes before Ekko Deathknell to ready all runes to double them?

Yes, you can tap (exhaust) all your runes for energy before Ekko's Deathknell trigger resolves, and then use the Deathknell effect to ready them.

According to Rule 163.2.a and Rule 163.2.b, basic runes have [Reaction] abilities to add energy or power. Because these are [Reaction] speed, you can activate them at any time you have priority, including while a Deathknell trigger is on the chain waiting to resolve.

Here is how the sequence works:

1. Ekko dies: His Deathknell triggers and is placed on the chain as a pending item (Rule 808.1.d.2).
2. Priority: Before the Deathknell trigger resolves, you have an opportunity to act. You can exhaust all your available runes to add energy to your pool.
3. Resolution: The Deathknell trigger resolves. You pay the cost (recycling Ekko) and the effect readies your runes (Rule 808.1.c).
4. Double Dip: Because your runes are now ready again, you can exhaust them a second time to add more energy to your pool for the rest of the turn.

Important Nuances:

• Recycling is a cost: As noted in FAQ #1510, the "recycle me" portion of Ekko's Deathknell is a mandatory cost paid at resolution. You must be able to recycle Ekko to ready the runes.
• No "Doubling" of the trigger: You cannot trigger Ekko's Deathknell twice from a single death (e.g., with Karthus), because you cannot recycle the same card twice (FAQ #1510, FAQ #7490).
• Timing: You can perform these actions as long as you have priority. If the Deathknell was triggered by a spell or ability, you are reacting to that item on the chain.

This is a standard play to maximize your total energy for the turn.